6. The IoC container

6.1 Introduction to the Spring IoC container and beans

This chapter covers the Spring Framework implementation of the Inversion of Control
(IoC) [1] principle. IoC
is also known as dependency injection (DI). It is a process whereby objects define
their dependencies, that is, the other objects they work with, only through constructor
arguments, arguments to a factory method, or properties that are set on the object
instance after it is constructed or returned from a factory method. The container then
injects those dependencies when it creates the bean. This process is fundamentally
the inverse, hence the name Inversion of Control (IoC), of the bean itself
controlling the instantiation or location of its dependencies by using direct
construction of classes, or a mechanism such as the Service Locator pattern.

The org.springframework.beans and org.springframework.context packages are the basis
for Spring Framework’s IoC container. The
BeanFactory
interface provides an advanced configuration mechanism capable of managing any type of
object.
ApplicationContext
is a sub-interface of BeanFactory. It adds easier integration with Spring’s AOP
features; message resource handling (for use in internationalization), event
publication; and application-layer specific contexts such as the WebApplicationContext
for use in web applications.

In short, the BeanFactory provides the configuration framework and basic
functionality, and the ApplicationContext adds more enterprise-specific functionality.
The ApplicationContext is a complete superset of the BeanFactory, and is used
exclusively in this chapter in descriptions of Spring’s IoC container. For more
information on using the BeanFactory instead of the ApplicationContext, refer to
Section 6.16, “The BeanFactory”.

In Spring, the objects that form the backbone of your application and that are managed
by the Spring IoC container are called beans. A bean is an object that is
instantiated, assembled, and otherwise managed by a Spring IoC container. Otherwise, a
bean is simply one of many objects in your application. Beans, and the dependencies
among them, are reflected in the configuration metadata used by a container.

6.2 Container overview

The interface org.springframework.context.ApplicationContext represents the Spring IoC
container and is responsible for instantiating, configuring, and assembling the
aforementioned beans. The container gets its instructions on what objects to
instantiate, configure, and assemble by reading configuration metadata. The
configuration metadata is represented in XML, Java annotations, or Java code. It allows
you to express the objects that compose your application and the rich interdependencies
between such objects.

Several implementations of the ApplicationContext interface are supplied
out-of-the-box with Spring. In standalone applications it is common to create an
instance of
ClassPathXmlApplicationContext
or FileSystemXmlApplicationContext.
While XML has been the traditional format for defining configuration metadata you can
instruct the container to use Java annotations or code as the metadata format by
providing a small amount of XML configuration to declaratively enable support for these
additional metadata formats.

In most application scenarios, explicit user code is not required to instantiate one or
more instances of a Spring IoC container. For example, in a web application scenario, a
simple eight (or so) lines of boilerplate web descriptor XML in the web.xml file
of the application will typically suffice (see Section 6.15.4, “Convenient ApplicationContext instantiation for web applications”). If you are using the
Spring Tool Suite Eclipse-powered development
environment this boilerplate configuration can be easily created with few mouse clicks or
keystrokes.

The following diagram is a high-level view of how Spring works. Your application classes
are combined with configuration metadata so that after the ApplicationContext is
created and initialized, you have a fully configured and executable system or
application.

Figure 6.1. The Spring IoC container

6.2.1 Configuration metadata

As the preceding diagram shows, the Spring IoC container consumes a form of
configuration metadata; this configuration metadata represents how you as an
application developer tell the Spring container to instantiate, configure, and assemble
the objects in your application.

Configuration metadata is traditionally supplied in a simple and intuitive XML format,
which is what most of this chapter uses to convey key concepts and features of the
Spring IoC container.

Note

XML-based metadata is not the only allowed form of configuration metadata. The
Spring IoC container itself is totally decoupled from the format in which this
configuration metadata is actually written. These days many developers choose
Java-based configuration for their Spring applications.

For information about using other forms of metadata with the Spring container, see:

Java-based configuration: Starting with Spring 3.0, many features
provided by the Spring JavaConfig project became part of the core Spring Framework.
Thus you can define beans external to your application classes by using Java rather
than XML files. To use these new features, see the @Configuration, @Bean, @Import
and @DependsOn annotations.

Spring configuration consists of at least one and typically more than one bean
definition that the container must manage. XML-based configuration metadata shows these
beans configured as <bean/> elements inside a top-level <beans/> element. Java
configuration typically uses @Bean annotated methods within a @Configuration class.

These bean definitions correspond to the actual objects that make up your application.
Typically you define service layer objects, data access objects (DAOs), presentation
objects such as Struts Action instances, infrastructure objects such as Hibernate
SessionFactories, JMS Queues, and so forth. Typically one does not configure
fine-grained domain objects in the container, because it is usually the responsibility
of DAOs and business logic to create and load domain objects. However, you can use
Spring’s integration with AspectJ to configure objects that have been created outside
the control of an IoC container. See Using AspectJ to
dependency-inject domain objects with Spring.

The following example shows the basic structure of XML-based configuration metadata:

The id attribute is a string that you use to identify the individual bean definition.
The class attribute defines the type of the bean and uses the fully qualified
classname. The value of the id attribute refers to collaborating objects. The XML for
referring to collaborating objects is not shown in this example; see
Dependencies for more information.

6.2.2 Instantiating a container

Instantiating a Spring IoC container is straightforward. The location path or paths
supplied to an ApplicationContext constructor are actually resource strings that allow
the container to load configuration metadata from a variety of external resources such
as the local file system, from the Java CLASSPATH, and so on.

After you learn about Spring’s IoC container, you may want to know more about Spring’s
Resource abstraction, as described in Chapter 7, Resources, which provides a convenient
mechanism for reading an InputStream from locations defined in a URI syntax. In
particular, Resource paths are used to construct applications contexts as described in
Section 7.7, “Application contexts and Resource paths”.

The following example shows the service layer objects (services.xml) configuration file:

In the preceding example, the service layer consists of the class PetStoreServiceImpl,
and two data access objects of the type JpaAccountDao and JpaItemDao (based
on the JPA Object/Relational mapping standard). The property name element refers to the
name of the JavaBean property, and the ref element refers to the name of another bean
definition. This linkage between id and ref elements expresses the dependency between
collaborating objects. For details of configuring an object’s dependencies, see
Dependencies.

Composing XML-based configuration metadata

It can be useful to have bean definitions span multiple XML files. Often each individual
XML configuration file represents a logical layer or module in your architecture.

You can use the application context constructor to load bean definitions from all these
XML fragments. This constructor takes multiple Resource locations, as was shown in the
previous section. Alternatively, use one or more occurrences of the <import/> element
to load bean definitions from another file or files. For example:

In the preceding example, external bean definitions are loaded from three files:
services.xml, messageSource.xml, and themeSource.xml. All location paths are
relative to the definition file doing the importing, so services.xml must be in the
same directory or classpath location as the file doing the importing, while
messageSource.xml and themeSource.xml must be in a resources location below the
location of the importing file. As you can see, a leading slash is ignored, but given
that these paths are relative, it is better form not to use the slash at all. The
contents of the files being imported, including the top level <beans/> element, must
be valid XML bean definitions according to the Spring Schema.

Note

It is possible, but not recommended, to reference files in parent directories using a
relative "../" path. Doing so creates a dependency on a file that is outside the current
application. In particular, this reference is not recommended for "classpath:" URLs (for
example, "classpath:../services.xml"), where the runtime resolution process chooses the
"nearest" classpath root and then looks into its parent directory. Classpath
configuration changes may lead to the choice of a different, incorrect directory.

You can always use fully qualified resource locations instead of relative paths: for
example, "file:C:/config/services.xml" or "classpath:/config/services.xml". However, be
aware that you are coupling your application’s configuration to specific absolute
locations. It is generally preferable to keep an indirection for such absolute
locations, for example, through "${…​}" placeholders that are resolved against JVM
system properties at runtime.

6.2.3 Using the container

The ApplicationContext is the interface for an advanced factory capable of maintaining
a registry of different beans and their dependencies. Using the method T getBean(String
name, Class<T> requiredType) you can retrieve instances of your beans.

The ApplicationContext enables you to read bean definitions and access them as follows:

You use getBean() to retrieve instances of your beans. The ApplicationContext
interface has a few other methods for retrieving beans, but ideally your application
code should never use them. Indeed, your application code should have no calls to the
getBean() method at all, and thus no dependency on Spring APIs at all. For example,
Spring’s integration with web frameworks provides for dependency injection for various
web framework classes such as controllers and JSF-managed beans.

6.3 Bean overview

A Spring IoC container manages one or more beans. These beans are created with the
configuration metadata that you supply to the container, for example, in the form of XML
<bean/> definitions.

Within the container itself, these bean definitions are represented as BeanDefinition
objects, which contain (among other information) the following metadata:

A package-qualified class name: typically the actual implementation class of the
bean being defined.

Bean behavioral configuration elements, which state how the bean should behave in the
container (scope, lifecycle callbacks, and so forth).

References to other beans that are needed for the bean to do its work; these
references are also called collaborators or dependencies.

Other configuration settings to set in the newly created object, for example, the
number of connections to use in a bean that manages a connection pool, or the size
limit of the pool.

This metadata translates to a set of properties that make up each bean definition.

In addition to bean definitions that contain information on how to create a specific
bean, the ApplicationContext implementations also permit the registration of existing
objects that are created outside the container, by users. This is done by accessing the
ApplicationContext’s BeanFactory via the method getBeanFactory() which returns the
BeanFactory implementation DefaultListableBeanFactory. DefaultListableBeanFactory
supports this registration through the methods registerSingleton(..) and
registerBeanDefinition(..). However, typical applications work solely with beans
defined through metadata bean definitions.

Note

Bean metadata and manually supplied singleton instances need to be registered as early
as possible, in order for the container to properly reason about them during autowiring
and other introspection steps. While overriding of existing metadata and existing
singleton instances is supported to some degree, the registration of new beans at
runtime (concurrently with live access to factory) is not officially supported and may
lead to concurrent access exceptions and/or inconsistent state in the bean container.

6.3.1 Naming beans

Every bean has one or more identifiers. These identifiers must be unique within the
container that hosts the bean. A bean usually has only one identifier, but if it
requires more than one, the extra ones can be considered aliases.

In XML-based configuration metadata, you use the id and/or name attributes
to specify the bean identifier(s). The id attribute allows you to specify
exactly one id. Conventionally these names are alphanumeric ('myBean',
'fooService', etc.), but may contain special characters as well. If you want to
introduce other aliases to the bean, you can also specify them in the name
attribute, separated by a comma (,), semicolon (;), or white space. As a
historical note, in versions prior to Spring 3.1, the id attribute was
defined as an xsd:ID type, which constrained possible characters. As of 3.1,
it is defined as an xsd:string type. Note that bean id uniqueness is still
enforced by the container, though no longer by XML parsers.

You are not required to supply a name or id for a bean. If no name or id is supplied
explicitly, the container generates a unique name for that bean. However, if you want to
refer to that bean by name, through the use of the ref element or
Service Locator style lookup, you must provide a name.
Motivations for not supplying a name are related to using inner
beans and autowiring collaborators.

Bean Naming Conventions

The convention is to use the standard Java convention for instance field names when
naming beans. That is, bean names start with a lowercase letter, and are camel-cased
from then on. Examples of such names would be (without quotes) 'accountManager',
'accountService', 'userDao', 'loginController', and so forth.

Naming beans consistently makes your configuration easier to read and understand, and if
you are using Spring AOP it helps a lot when applying advice to a set of beans related
by name.

Note

With component scanning in the classpath, Spring generates bean names for unnamed
components, following the rules above: essentially, taking the simple class name
and turning its initial character to lower-case. However, in the (unusual) special
case when there is more than one character and both the first and second characters
are upper case, the original casing gets preserved. These are the same rules as
defined by java.beans.Introspector.decapitalize (which Spring is using here).

Aliasing a bean outside the bean definition

In a bean definition itself, you can supply more than one name for the bean, by using a
combination of up to one name specified by the id attribute, and any number of other
names in the name attribute. These names can be equivalent aliases to the same bean,
and are useful for some situations, such as allowing each component in an application to
refer to a common dependency by using a bean name that is specific to that component
itself.

Specifying all aliases where the bean is actually defined is not always adequate,
however. It is sometimes desirable to introduce an alias for a bean that is defined
elsewhere. This is commonly the case in large systems where configuration is split
amongst each subsystem, each subsystem having its own set of object definitions. In
XML-based configuration metadata, you can use the <alias/> element to accomplish this.

<aliasname="fromName"alias="toName"/>

In this case, a bean in the same container which is named fromName, may also,
after the use of this alias definition, be referred to as toName.

For example, the configuration metadata for subsystem A may refer to a DataSource via
the name subsystemA-dataSource. The configuration metadata for subsystem B may refer to
a DataSource via the name subsystemB-dataSource. When composing the main application
that uses both these subsystems the main application refers to the DataSource via the
name myApp-dataSource. To have all three names refer to the same object you add to the
MyApp configuration metadata the following aliases definitions:

Now each component and the main application can refer to the dataSource through a name
that is unique and guaranteed not to clash with any other definition (effectively
creating a namespace), yet they refer to the same bean.

6.3.2 Instantiating beans

A bean definition essentially is a recipe for creating one or more objects. The
container looks at the recipe for a named bean when asked, and uses the configuration
metadata encapsulated by that bean definition to create (or acquire) an actual object.

Typically, to specify the bean class to be constructed in the case where the container
itself directly creates the bean by calling its constructor reflectively, somewhat
equivalent to Java code using the new operator.

To specify the actual class containing the static factory method that will be
invoked to create the object, in the less common case where the container invokes a
staticfactory method on a class to create the bean. The object type returned
from the invocation of the static factory method may be the same class or another
class entirely.

Inner class names.
If you want to configure a bean definition for a static nested class, you have to use
the binary name of the nested class.

For example, if you have a class called Foo in the com.example package, and this
Foo class has a static nested class called Bar, the value of the 'class'
attribute on a bean definition would be…​

com.example.Foo$Bar

Notice the use of the $ character in the name to separate the nested class name from
the outer class name.

Instantiation with a constructor

When you create a bean by the constructor approach, all normal classes are usable by and
compatible with Spring. That is, the class being developed does not need to implement
any specific interfaces or to be coded in a specific fashion. Simply specifying the bean
class should suffice. However, depending on what type of IoC you use for that specific
bean, you may need a default (empty) constructor.

The Spring IoC container can manage virtually any class you want it to manage; it is
not limited to managing true JavaBeans. Most Spring users prefer actual JavaBeans with
only a default (no-argument) constructor and appropriate setters and getters modeled
after the properties in the container. You can also have more exotic non-bean-style
classes in your container. If, for example, you need to use a legacy connection pool
that absolutely does not adhere to the JavaBean specification, Spring can manage it as
well.

With XML-based configuration metadata you can specify your bean class as follows:

For details about the mechanism for supplying arguments to the constructor (if required)
and setting object instance properties after the object is constructed, see
Injecting Dependencies.

Instantiation with a static factory method

When defining a bean that you create with a static factory method, you use the class
attribute to specify the class containing the static factory method and an attribute
named factory-method to specify the name of the factory method itself. You should be
able to call this method (with optional arguments as described later) and return a live
object, which subsequently is treated as if it had been created through a constructor.
One use for such a bean definition is to call static factories in legacy code.

The following bean definition specifies that the bean will be created by calling a
factory-method. The definition does not specify the type (class) of the returned object,
only the class containing the factory method. In this example, the createInstance()
method must be a static method.

For details about the mechanism for supplying (optional) arguments to the factory method
and setting object instance properties after the object is returned from the factory,
see Dependencies and configuration in detail.

Instantiation using an instance factory method

Similar to instantiation through a static
factory method, instantiation with an instance factory method invokes a non-static
method of an existing bean from the container to create a new bean. To use this
mechanism, leave the class attribute empty, and in the factory-bean attribute,
specify the name of a bean in the current (or parent/ancestor) container that contains
the instance method that is to be invoked to create the object. Set the name of the
factory method itself with the factory-method attribute.

<!-- the factory bean, which contains a method called createInstance() --><beanid="serviceLocator"class="examples.DefaultServiceLocator"><!-- inject any dependencies required by this locator bean --></bean><!-- the bean to be created via the factory bean --><beanid="clientService"factory-bean="serviceLocator"factory-method="createClientServiceInstance"/>

In Spring documentation, factory bean refers to a bean that is configured in the
Spring container that will create objects through an
instance or
static factory method. By contrast,
FactoryBean (notice the capitalization) refers to a Spring-specific
FactoryBean.

6.4 Dependencies

A typical enterprise application does not consist of a single object (or bean in the
Spring parlance). Even the simplest application has a few objects that work together to
present what the end-user sees as a coherent application. This next section explains how
you go from defining a number of bean definitions that stand alone to a fully realized
application where objects collaborate to achieve a goal.

6.4.1 Dependency Injection

Dependency injection (DI) is a process whereby objects define their dependencies,
that is, the other objects they work with, only through constructor arguments, arguments
to a factory method, or properties that are set on the object instance after it is
constructed or returned from a factory method. The container then injects those
dependencies when it creates the bean. This process is fundamentally the inverse, hence
the name Inversion of Control (IoC), of the bean itself controlling the instantiation
or location of its dependencies on its own by using direct construction of classes, or
the Service Locator pattern.

Code is cleaner with the DI principle and decoupling is more effective when objects are
provided with their dependencies. The object does not look up its dependencies, and does
not know the location or class of the dependencies. As such, your classes become easier
to test, in particular when the dependencies are on interfaces or abstract base classes,
which allow for stub or mock implementations to be used in unit tests.

Constructor-based dependency injection

Constructor-based DI is accomplished by the container invoking a constructor with a
number of arguments, each representing a dependency. Calling a static factory method
with specific arguments to construct the bean is nearly equivalent, and this discussion
treats arguments to a constructor and to a static factory method similarly. The
following example shows a class that can only be dependency-injected with constructor
injection. Notice that there is nothing special about this class, it is a POJO that
has no dependencies on container specific interfaces, base classes or annotations.

publicclass SimpleMovieLister {
// the SimpleMovieLister has a dependency on a MovieFinderprivate MovieFinder movieFinder;
// a constructor so that the Spring container can inject a MovieFinderpublic SimpleMovieLister(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// business logic that actually uses the injected MovieFinder is omitted...
}

Constructor argument resolution

Constructor argument resolution matching occurs using the argument’s type. If no
potential ambiguity exists in the constructor arguments of a bean definition, then the
order in which the constructor arguments are defined in a bean definition is the order
in which those arguments are supplied to the appropriate constructor when the bean is
being instantiated. Consider the following class:

No potential ambiguity exists, assuming that Bar and Baz classes are not related by
inheritance. Thus the following configuration works fine, and you do not need to specify
the constructor argument indexes and/or types explicitly in the <constructor-arg/>
element.

When another bean is referenced, the type is known, and matching can occur (as was the
case with the preceding example). When a simple type is used, such as
<value>true</value>, Spring cannot determine the type of the value, and so cannot match
by type without help. Consider the following class:

Keep in mind that to make this work out of the box your code must be compiled with the
debug flag enabled so that Spring can look up the parameter name from the constructor.
If you can’t compile your code with debug flag (or don’t want to) you can use
@ConstructorProperties
JDK annotation to explicitly name your constructor arguments. The sample class would
then have to look as follows:

Setter-based dependency injection

The following example shows a class that can only be dependency-injected using pure
setter injection. This class is conventional Java. It is a POJO that has no dependencies
on container specific interfaces, base classes or annotations.

publicclass SimpleMovieLister {
// the SimpleMovieLister has a dependency on the MovieFinderprivate MovieFinder movieFinder;
// a setter method so that the Spring container can inject a MovieFinderpublicvoid setMovieFinder(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// business logic that actually uses the injected MovieFinder is omitted...
}

The ApplicationContext supports constructor-based and setter-based DI for the beans it
manages. It also supports setter-based DI after some dependencies have already been
injected through the constructor approach. You configure the dependencies in the form of
a BeanDefinition, which you use in conjunction with PropertyEditor instances to
convert properties from one format to another. However, most Spring users do not work
with these classes directly (i.e., programmatically) but rather with XML bean
definitions, annotated components (i.e., classes annotated with @Component,
@Controller, etc.), or @Bean methods in Java-based @Configuration classes. These
sources are then converted internally into instances of BeanDefinition and used to
load an entire Spring IoC container instance.

Constructor-based or setter-based DI?

Since you can mix constructor-based and setter-based DI, it is a good rule of thumb to
use constructors for mandatory dependencies and setter methods or configuration methods
for optional dependencies. Note that use of the @Required
annotation on a setter method can be used to make the property a required dependency.

The Spring team generally advocates constructor injection as it enables one to implement
application components as immutable objects and to ensure that required dependencies
are not null. Furthermore constructor-injected components are always returned to client
(calling) code in a fully initialized state. As a side note, a large number of constructor
arguments is a bad code smell, implying that the class likely has too many
responsibilities and should be refactored to better address proper separation of concerns.

Setter injection should primarily only be used for optional dependencies that can be
assigned reasonable default values within the class. Otherwise, not-null checks must be
performed everywhere the code uses the dependency. One benefit of setter injection is that
setter methods make objects of that class amenable to reconfiguration or re-injection
later. Management through JMX MBeans is therefore a compelling use case for setter
injection.

Use the DI style that makes the most sense for a particular class. Sometimes, when dealing
with third-party classes for which you do not have the source, the choice is made for you.
For example, if a third-party class does not expose any setter methods, then constructor
injection may be the only available form of DI.

Dependency resolution process

The container performs bean dependency resolution as follows:

The ApplicationContext is created and initialized with configuration metadata that
describes all the beans. Configuration metadata can be specified via XML, Java code, or
annotations.

For each bean, its dependencies are expressed in the form of properties, constructor
arguments, or arguments to the static-factory method if you are using that instead of
a normal constructor. These dependencies are provided to the bean, when the bean is
actually created.

Each property or constructor argument is an actual definition of the value to set, or
a reference to another bean in the container.

Each property or constructor argument which is a value is converted from its specified
format to the actual type of that property or constructor argument. By default Spring
can convert a value supplied in string format to all built-in types, such as int,
long, String, boolean, etc.

The Spring container validates the configuration of each bean as the container is created.
However, the bean properties themselves are not set until the bean is actually created.
Beans that are singleton-scoped and set to be pre-instantiated (the default) are created
when the container is created. Scopes are defined in Section 6.5, “Bean scopes”. Otherwise,
the bean is created only when it is requested. Creation of a bean potentially causes a
graph of beans to be created, as the bean’s dependencies and its dependencies'
dependencies (and so on) are created and assigned. Note that resolution mismatches among
those dependencies may show up late, i.e. on first creation of the affected bean.

Circular dependencies

If you use predominantly constructor injection, it is possible to create an unresolvable
circular dependency scenario.

For example: Class A requires an instance of class B through constructor injection, and
class B requires an instance of class A through constructor injection. If you configure
beans for classes A and B to be injected into each other, the Spring IoC container
detects this circular reference at runtime, and throws a
BeanCurrentlyInCreationException.

One possible solution is to edit the source code of some classes to be configured by
setters rather than constructors. Alternatively, avoid constructor injection and use
setter injection only. In other words, although it is not recommended, you can configure
circular dependencies with setter injection.

Unlike the typical case (with no circular dependencies), a circular dependency
between bean A and bean B forces one of the beans to be injected into the other prior to
being fully initialized itself (a classic chicken/egg scenario).

You can generally trust Spring to do the right thing. It detects configuration problems,
such as references to non-existent beans and circular dependencies, at container
load-time. Spring sets properties and resolves dependencies as late as possible, when
the bean is actually created. This means that a Spring container which has loaded
correctly can later generate an exception when you request an object if there is a
problem creating that object or one of its dependencies. For example, the bean throws an
exception as a result of a missing or invalid property. This potentially delayed
visibility of some configuration issues is why ApplicationContext implementations by
default pre-instantiate singleton beans. At the cost of some upfront time and memory to
create these beans before they are actually needed, you discover configuration issues
when the ApplicationContext is created, not later. You can still override this default
behavior so that singleton beans will lazy-initialize, rather than be pre-instantiated.

If no circular dependencies exist, when one or more collaborating beans are being
injected into a dependent bean, each collaborating bean is totally configured prior
to being injected into the dependent bean. This means that if bean A has a dependency on
bean B, the Spring IoC container completely configures bean B prior to invoking the
setter method on bean A. In other words, the bean is instantiated (if not a
pre-instantiated singleton), its dependencies are set, and the relevant lifecycle
methods (such as a configured init method
or the InitializingBean callback method)
are invoked.

Examples of dependency injection

The following example uses XML-based configuration metadata for setter-based DI. A small
part of a Spring XML configuration file specifies some bean definitions:

publicclass ExampleBean {
// a private constructorprivate ExampleBean(...) {
...
}
// a static factory method; the arguments to this method can be// considered the dependencies of the bean that is returned,// regardless of how those arguments are actually used.publicstatic ExampleBean createInstance (
AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {
ExampleBean eb = new ExampleBean (...);
// some other operations...return eb;
}
}

Arguments to the static factory method are supplied via <constructor-arg/> elements,
exactly the same as if a constructor had actually been used. The type of the class being
returned by the factory method does not have to be of the same type as the class that
contains the static factory method, although in this example it is. An instance
(non-static) factory method would be used in an essentially identical fashion (aside
from the use of the factory-bean attribute instead of the class attribute), so
details will not be discussed here.

6.4.2 Dependencies and configuration in detail

As mentioned in the previous section, you can define bean properties and constructor
arguments as references to other managed beans (collaborators), or as values defined
inline. Spring’s XML-based configuration metadata supports sub-element types within its
<property/> and <constructor-arg/> elements for this purpose.

Straight values (primitives, Strings, and so on)

The value attribute of the <property/> element specifies a property or constructor
argument as a human-readable string representation. Spring’s
conversion service is used to convert these
values from a String to the actual type of the property or argument.

The preceding XML is more succinct; however, typos are discovered at runtime rather than
design time, unless you use an IDE such as IntelliJ
IDEA or the Spring Tool Suite (STS)
that support automatic property completion when you create bean definitions. Such IDE
assistance is highly recommended.

The Spring container converts the text inside the <value/> element into a
java.util.Properties instance by using the JavaBeans PropertyEditor mechanism. This
is a nice shortcut, and is one of a few places where the Spring team do favor the use of
the nested <value/> element over the value attribute style.

The idref element

The idref element is simply an error-proof way to pass the id (string value - not
a reference) of another bean in the container to a <constructor-arg/> or <property/>
element.

The first form is preferable to the second, because using the idref tag allows the
container to validate at deployment time that the referenced, named bean actually
exists. In the second variation, no validation is performed on the value that is passed
to the targetName property of the client bean. Typos are only discovered (with most
likely fatal results) when the client bean is actually instantiated. If the client
bean is a prototype bean, this typo and the resulting exception
may only be discovered long after the container is deployed.

Note

The local attribute on the idref element is no longer supported in the 4.0 beans xsd
since it does not provide value over a regular bean reference anymore. Simply change
your existing idref local references to idref bean when upgrading to the 4.0 schema.

A common place (at least in versions earlier than Spring 2.0) where the <idref/> element
brings value is in the configuration of AOP interceptors in a
ProxyFactoryBean bean definition. Using <idref/> elements when you specify the
interceptor names prevents you from misspelling an interceptor id.

References to other beans (collaborators)

The ref element is the final element inside a <constructor-arg/> or <property/>
definition element. Here you set the value of the specified property of a bean to be a
reference to another bean (a collaborator) managed by the container. The referenced bean
is a dependency of the bean whose property will be set, and it is initialized on demand
as needed before the property is set. (If the collaborator is a singleton bean, it may
be initialized already by the container.) All references are ultimately a reference to
another object. Scoping and validation depend on whether you specify the id/name of the
other object through the bean, local, or parent attributes.

Specifying the target bean through the bean attribute of the <ref/> tag is the most
general form, and allows creation of a reference to any bean in the same container or
parent container, regardless of whether it is in the same XML file. The value of the
bean attribute may be the same as the id attribute of the target bean, or as one of
the values in the name attribute of the target bean.

<refbean="someBean"/>

Specifying the target bean through the parent attribute creates a reference to a bean
that is in a parent container of the current container. The value of the parent
attribute may be the same as either the id attribute of the target bean, or one of the
values in the name attribute of the target bean, and the target bean must be in a
parent container of the current one. You use this bean reference variant mainly when you
have a hierarchy of containers and you want to wrap an existing bean in a parent
container with a proxy that will have the same name as the parent bean.

<!-- in the parent context --><beanid="accountService"class="com.foo.SimpleAccountService"><!-- insert dependencies as required as here --></bean>

<!-- in the child (descendant) context --><beanid="accountService"<!--beannameisthesameastheparentbean-->class="org.springframework.aop.framework.ProxyFactoryBean"><propertyname="target"><refparent="accountService"/><!-- notice how we refer to the parent bean --></property><!-- insert other configuration and dependencies as required here --></bean>

Note

The local attribute on the ref element is no longer supported in the 4.0 beans xsd
since it does not provide value over a regular bean reference anymore. Simply change
your existing ref local references to ref bean when upgrading to the 4.0 schema.

Inner beans

<beanid="outer"class="..."><!-- instead of using a reference to a target bean, simply define the target bean inline --><propertyname="target"><beanclass="com.example.Person"><!-- this is the inner bean --><propertyname="name"value="Fiona Apple"/><propertyname="age"value="25"/></bean></property></bean>

An inner bean definition does not require a defined id or name; if specified, the container
does not use such a value as an identifier. The container also ignores the scope flag on
creation: Inner beans are always anonymous and they are always created with the outer
bean. It is not possible to inject inner beans into collaborating beans other than into
the enclosing bean or to access them independently.

As a corner case, it is possible to receive destruction callbacks from a custom scope, e.g.
for a request-scoped inner bean contained within a singleton bean: The creation of the inner
bean instance will be tied to its containing bean, but destruction callbacks allow it to
participate in the request scope’s lifecycle. This is not a common scenario; inner beans
typically simply share their containing bean’s scope.

Collections

In the <list/>, <set/>, <map/>, and <props/> elements, you set the properties
and arguments of the Java Collection types List, Set, Map, and Properties,
respectively.

<beanid="moreComplexObject"class="example.ComplexObject"><!-- results in a setAdminEmails(java.util.Properties) call --><propertyname="adminEmails"><props><propkey="administrator">[email protected]</prop><propkey="support">[email protected]</prop><propkey="development">[email protected]</prop></props></property><!-- results in a setSomeList(java.util.List) call --><propertyname="someList"><list><value>a list element followed by a reference</value><refbean="myDataSource" /></list></property><!-- results in a setSomeMap(java.util.Map) call --><propertyname="someMap"><map><entrykey="an entry"value="just some string"/><entrykey ="a ref"value-ref="myDataSource"/></map></property><!-- results in a setSomeSet(java.util.Set) call --><propertyname="someSet"><set><value>just some string</value><refbean="myDataSource" /></set></property></bean>

The value of a map key or value, or a set value, can also again be any of the
following elements:

bean | ref | idref | list | set | map | props | value | null

Collection merging

The Spring container also supports the merging of collections. An application
developer can define a parent-style <list/>, <map/>, <set/> or <props/> element,
and have child-style <list/>, <map/>, <set/> or <props/> elements inherit and
override values from the parent collection. That is, the child collection’s values are
the result of merging the elements of the parent and child collections, with the child’s
collection elements overriding values specified in the parent collection.

This section on merging discusses the parent-child bean mechanism. Readers unfamiliar
with parent and child bean definitions may wish to read the
relevant section before continuing.

Notice the use of the merge=true attribute on the <props/> element of the
adminEmails property of the child bean definition. When the child bean is resolved
and instantiated by the container, the resulting instance has an adminEmailsProperties collection that contains the result of the merging of the child’s
adminEmails collection with the parent’s adminEmails collection.

The child Properties collection’s value set inherits all property elements from the
parent <props/>, and the child’s value for the support value overrides the value in
the parent collection.

This merging behavior applies similarly to the <list/>, <map/>, and <set/>
collection types. In the specific case of the <list/> element, the semantics
associated with the List collection type, that is, the notion of an ordered
collection of values, is maintained; the parent’s values precede all of the child list’s
values. In the case of the Map, Set, and Properties collection types, no ordering
exists. Hence no ordering semantics are in effect for the collection types that underlie
the associated Map, Set, and Properties implementation types that the container
uses internally.

Limitations of collection merging

You cannot merge different collection types (such as a Map and a List), and if you
do attempt to do so an appropriate Exception is thrown. The merge attribute must be
specified on the lower, inherited, child definition; specifying the merge attribute on
a parent collection definition is redundant and will not result in the desired merging.

Strongly-typed collection

With the introduction of generic types in Java 5, you can use strongly typed collections.
That is, it is possible to declare a Collection type such that it can only contain
String elements (for example). If you are using Spring to dependency-inject a
strongly-typed Collection into a bean, you can take advantage of Spring’s
type-conversion support such that the elements of your strongly-typed Collection
instances are converted to the appropriate type prior to being added to the Collection.

When the accounts property of the foo bean is prepared for injection, the generics
information about the element type of the strongly-typed Map<String, Float> is
available by reflection. Thus Spring’s type conversion infrastructure recognizes the
various value elements as being of type Float, and the string values 9.99, 2.75, and
3.99 are converted into an actual Float type.

Null and empty string values

Spring treats empty arguments for properties and the like as empty Strings. The
following XML-based configuration metadata snippet sets the email property to the empty
String value ("").

XML shortcut with the p-namespace

The p-namespace enables you to use the bean element’s attributes, instead of nested
<property/> elements, to describe your property values and/or collaborating beans.

Spring supports extensible configuration formats with namespaces, which are
based on an XML Schema definition. The beans configuration format discussed in this
chapter is defined in an XML Schema document. However, the p-namespace is not defined in
an XSD file and exists only in the core of Spring.

The following example shows two XML snippets that resolve to the same result: The first
uses standard XML format and the second uses the p-namespace.

The example shows an attribute in the p-namespace called email in the bean definition.
This tells Spring to include a property declaration. As previously mentioned, the
p-namespace does not have a schema definition, so you can set the name of the attribute
to the property name.

This next example includes two more bean definitions that both have a reference to
another bean:

As you can see, this example includes not only a property value using the p-namespace,
but also uses a special format to declare property references. Whereas the first bean
definition uses <property name="spouse" ref="jane"/> to create a reference from bean
john to bean jane, the second bean definition uses p:spouse-ref="jane" as an
attribute to do the exact same thing. In this case spouse is the property name,
whereas the -ref part indicates that this is not a straight value but rather a
reference to another bean.

Note

The p-namespace is not as flexible as the standard XML format. For example, the format
for declaring property references clashes with properties that end in Ref, whereas the
standard XML format does not. We recommend that you choose your approach carefully and
communicate this to your team members, to avoid producing XML documents that use all
three approaches at the same time.

The c: namespace uses the same conventions as the p: one (trailing -ref for bean
references) for setting the constructor arguments by their names. And just as well, it
needs to be declared even though it is not defined in an XSD schema (but it exists
inside the Spring core).

For the rare cases where the constructor argument names are not available (usually if
the bytecode was compiled without debugging information), one can use fallback to the
argument indexes:

The foo bean has a fred property, which has a bob property, which has a sammy
property, and that final sammy property is being set to the value 123. In order for
this to work, the fred property of foo, and the bob property of fred must not be
null after the bean is constructed, or a NullPointerException is thrown.

6.4.3 Using depends-on

If a bean is a dependency of another that usually means that one bean is set as a
property of another. Typically you accomplish this with the <ref/>
element in XML-based configuration metadata. However, sometimes dependencies between
beans are less direct; for example, a static initializer in a class needs to be
triggered, such as database driver registration. The depends-on attribute can
explicitly force one or more beans to be initialized before the bean using this element
is initialized. The following example uses the depends-on attribute to express a
dependency on a single bean:

The depends-on attribute in the bean definition can specify both an initialization
time dependency and, in the case of singleton beans
only, a corresponding destroy time dependency. Dependent beans that define a
depends-on relationship with a given bean are destroyed first, prior to the given bean
itself being destroyed. Thus depends-on can also control shutdown order.

6.4.4 Lazy-initialized beans

By default, ApplicationContext implementations eagerly create and configure all
singleton beans as part of the initialization
process. Generally, this pre-instantiation is desirable, because errors in the
configuration or surrounding environment are discovered immediately, as opposed to hours
or even days later. When this behavior is not desirable, you can prevent
pre-instantiation of a singleton bean by marking the bean definition as
lazy-initialized. A lazy-initialized bean tells the IoC container to create a bean
instance when it is first requested, rather than at startup.

In XML, this behavior is controlled by the lazy-init attribute on the <bean/>
element; for example:

When the preceding configuration is consumed by an ApplicationContext, the bean named
lazy is not eagerly pre-instantiated when the ApplicationContext is starting up,
whereas the not.lazy bean is eagerly pre-instantiated.

However, when a lazy-initialized bean is a dependency of a singleton bean that is
not lazy-initialized, the ApplicationContext creates the lazy-initialized bean at
startup, because it must satisfy the singleton’s dependencies. The lazy-initialized bean
is injected into a singleton bean elsewhere that is not lazy-initialized.

You can also control lazy-initialization at the container level by using the
default-lazy-init attribute on the <beans/> element; for example:

<beansdefault-lazy-init="true"><!-- no beans will be pre-instantiated... --></beans>

6.4.5 Autowiring collaborators

The Spring container can autowire relationships between collaborating beans. You can
allow Spring to resolve collaborators (other beans) automatically for your bean by
inspecting the contents of the ApplicationContext. Autowiring has the following
advantages:

Autowiring can significantly reduce the need to specify properties or constructor
arguments. (Other mechanisms such as a bean template
discussed elsewhere in this chapter are also valuable
in this regard.)

Autowiring can update a configuration as your objects evolve. For example, if you need
to add a dependency to a class, that dependency can be satisfied automatically without
you needing to modify the configuration. Thus autowiring can be especially useful
during development, without negating the option of switching to explicit wiring when
the code base becomes more stable.

When using XML-based configuration metadata [2], you specify autowire
mode for a bean definition with the autowire attribute of the <bean/> element. The
autowiring functionality has four modes. You specify autowiring per bean and thus
can choose which ones to autowire.

Table 6.2. Autowiring modes

Mode

Explanation

no

(Default) No autowiring. Bean references must be defined via a ref element. Changing
the default setting is not recommended for larger deployments, because specifying
collaborators explicitly gives greater control and clarity. To some extent, it
documents the structure of a system.

byName

Autowiring by property name. Spring looks for a bean with the same name as the
property that needs to be autowired. For example, if a bean definition is set to
autowire by name, and it contains a master property (that is, it has a
setMaster(..) method), Spring looks for a bean definition named master, and uses
it to set the property.

byType

Allows a property to be autowired if exactly one bean of the property type exists in
the container. If more than one exists, a fatal exception is thrown, which indicates
that you may not use byType autowiring for that bean. If there are no matching
beans, nothing happens; the property is not set.

constructor

Analogous to byType, but applies to constructor arguments. If there is not exactly
one bean of the constructor argument type in the container, a fatal error is raised.

With byType or constructor autowiring mode, you can wire arrays and
typed-collections. In such cases all autowire candidates within the container that
match the expected type are provided to satisfy the dependency. You can autowire
strongly-typed Maps if the expected key type is String. An autowired Maps values will
consist of all bean instances that match the expected type, and the Maps keys will
contain the corresponding bean names.

You can combine autowire behavior with dependency checking, which is performed after
autowiring completes.

Limitations and disadvantages of autowiring

Autowiring works best when it is used consistently across a project. If autowiring is
not used in general, it might be confusing to developers to use it to wire only one or
two bean definitions.

Consider the limitations and disadvantages of autowiring:

Explicit dependencies in property and constructor-arg settings always override
autowiring. You cannot autowire so-called simple properties such as primitives,
Strings, and Classes (and arrays of such simple properties). This limitation is
by-design.

Autowiring is less exact than explicit wiring. Although, as noted in the above table,
Spring is careful to avoid guessing in case of ambiguity that might have unexpected
results, the relationships between your Spring-managed objects are no longer
documented explicitly.

Wiring information may not be available to tools that may generate documentation from
a Spring container.

Multiple bean definitions within the container may match the type specified by the
setter method or constructor argument to be autowired. For arrays, collections, or
Maps, this is not necessarily a problem. However for dependencies that expect a single
value, this ambiguity is not arbitrarily resolved. If no unique bean definition is
available, an exception is thrown.

In the latter scenario, you have several options:

Abandon autowiring in favor of explicit wiring.

Avoid autowiring for a bean definition by setting its autowire-candidate attributes
to false as described in the next section.

Designate a single bean definition as the primary candidate by setting the
primary attribute of its <bean/> element to true.

Excluding a bean from autowiring

On a per-bean basis, you can exclude a bean from autowiring. In Spring’s XML format, set
the autowire-candidate attribute of the <bean/> element to false; the container
makes that specific bean definition unavailable to the autowiring infrastructure
(including annotation style configurations such as @Autowired).

You can also limit autowire candidates based on pattern-matching against bean names. The
top-level <beans/> element accepts one or more patterns within its
default-autowire-candidates attribute. For example, to limit autowire candidate status
to any bean whose name ends with Repository, provide a value of *Repository. To
provide multiple patterns, define them in a comma-separated list. An explicit value of
true or false for a bean definitions autowire-candidate attribute always takes
precedence, and for such beans, the pattern matching rules do not apply.

These techniques are useful for beans that you never want to be injected into other
beans by autowiring. It does not mean that an excluded bean cannot itself be configured
using autowiring. Rather, the bean itself is not a candidate for autowiring other beans.

6.4.6 Method injection

In most application scenarios, most beans in the container are
singletons. When a singleton bean needs to
collaborate with another singleton bean, or a non-singleton bean needs to collaborate
with another non-singleton bean, you typically handle the dependency by defining one
bean as a property of the other. A problem arises when the bean lifecycles are
different. Suppose singleton bean A needs to use non-singleton (prototype) bean B,
perhaps on each method invocation on A. The container only creates the singleton bean A
once, and thus only gets one opportunity to set the properties. The container cannot
provide bean A with a new instance of bean B every time one is needed.

The preceding is not desirable, because the business code is aware of and coupled to the
Spring Framework. Method Injection, a somewhat advanced feature of the Spring IoC
container, allows this use case to be handled in a clean fashion.

You can read more about the motivation for Method Injection in
this blog entry.

Lookup method injection

Lookup method injection is the ability of the container to override methods on
container managed beans, to return the lookup result for another named bean in the
container. The lookup typically involves a prototype bean as in the scenario described
in the preceding section. The Spring Framework implements this method injection by using
bytecode generation from the CGLIB library to generate dynamically a subclass that
overrides the method.

Note

For this dynamic subclassing to work, the class that the Spring bean container will
subclass cannot be final, and the method to be overridden cannot be final either.

Unit-testing a class that has an abstract method requires you to subclass the class
yourself and to supply a stub implementation of the abstract method.

Concrete methods are also necessary for component scanning which requires concrete
classes to pick up.

A further key limitation is that lookup methods won’t work with factory methods and
in particular not with @Bean methods in configuration classes, since the container
is not in charge of creating the instance in that case and therefore cannot create
a runtime-generated subclass on the fly.

Looking at the CommandManager class in the previous code snippet, you see that the
Spring container will dynamically override the implementation of the createCommand()
method. Your CommandManager class will not have any Spring dependencies, as can be
seen in the reworked example:

package fiona.apple;
// no more Spring imports!publicabstractclass CommandManager {
public Object process(Object commandState) {
// grab a new instance of the appropriate Command interface
Command command = createCommand();
// set the state on the (hopefully brand new) Command instance
command.setState(commandState);
return command.execute();
}
// okay... but where is the implementation of this method?protectedabstract Command createCommand();
}

In the client class containing the method to be injected (the CommandManager in this
case), the method to be injected requires a signature of the following form:

If the method is abstract, the dynamically-generated subclass implements the method.
Otherwise, the dynamically-generated subclass overrides the concrete method defined in
the original class. For example:

The bean identified as commandManager calls its own method createCommand()
whenever it needs a new instance of the myCommand bean. You must be careful to deploy
the myCommand bean as a prototype, if that is actually what is needed. If it is
as a singleton, the same instance of the myCommand
bean is returned each time.

Alternatively, within the annotation-based component model, you may declare a lookup
method through the @Lookup annotation:

Note that you will typically declare such annotated lookup methods with a concrete
stub implementation, in order for them to be compatible with Spring’s component
scanning rules where abstract classes get ignored by default. This limitation does not
apply in case of explicitly registered or explicitly imported bean classes.

The interested reader may also find the ServiceLocatorFactoryBean (in the
org.springframework.beans.factory.config package) to be of use.

Arbitrary method replacement

A less useful form of method injection than lookup method injection is the ability to
replace arbitrary methods in a managed bean with another method implementation. Users
may safely skip the rest of this section until the functionality is actually needed.

With XML-based configuration metadata, you can use the replaced-method element to
replace an existing method implementation with another, for a deployed bean. Consider
the following class, with a method computeValue, which we want to override:

You can use one or more contained <arg-type/> elements within the <replaced-method/>
element to indicate the method signature of the method being overridden. The signature
for the arguments is necessary only if the method is overloaded and multiple variants
exist within the class. For convenience, the type string for an argument may be a
substring of the fully qualified type name. For example, the following all match
java.lang.String:

java.lang.String
String
Str

Because the number of arguments is often enough to distinguish between each possible
choice, this shortcut can save a lot of typing, by allowing you to type only the
shortest string that will match an argument type.

6.5 Bean scopes

When you create a bean definition, you create a recipe for creating actual instances
of the class defined by that bean definition. The idea that a bean definition is a
recipe is important, because it means that, as with a class, you can create many object
instances from a single recipe.

You can control not only the various dependencies and configuration values that are to
be plugged into an object that is created from a particular bean definition, but also
the scope of the objects created from a particular bean definition. This approach is
powerful and flexible in that you can choose the scope of the objects you create
through configuration instead of having to bake in the scope of an object at the Java
class level. Beans can be defined to be deployed in one of a number of scopes: out of
the box, the Spring Framework supports seven scopes, five of which are available only if
you use a web-aware ApplicationContext.

The following scopes are supported out of the box. You can also create
a custom scope.

Scopes a single bean definition to the lifecycle of a single HTTP request; that is,
each HTTP request has its own instance of a bean created off the back of a single bean
definition. Only valid in the context of a web-aware Spring ApplicationContext.

6.5.1 The singleton scope

Only one shared instance of a singleton bean is managed, and all requests for beans
with an id or ids matching that bean definition result in that one specific bean
instance being returned by the Spring container.

To put it another way, when you define a bean definition and it is scoped as a
singleton, the Spring IoC container creates exactly one instance of the object
defined by that bean definition. This single instance is stored in a cache of such
singleton beans, and all subsequent requests and references for that named bean
return the cached object.

Spring’s concept of a singleton bean differs from the Singleton pattern as defined in
the Gang of Four (GoF) patterns book. The GoF Singleton hard-codes the scope of an
object such that one and only one instance of a particular class is created per
ClassLoader. The scope of the Spring singleton is best described as per container
and per bean. This means that if you define one bean for a particular class in a
single Spring container, then the Spring container creates one and only one instance
of the class defined by that bean definition. The singleton scope is the default scope
in Spring. To define a bean as a singleton in XML, you would write, for example:

<beanid="accountService"class="com.foo.DefaultAccountService"/><!-- the following is equivalent, though redundant (singleton scope is the default) --><beanid="accountService"class="com.foo.DefaultAccountService"scope="singleton"/>

6.5.2 The prototype scope

The non-singleton, prototype scope of bean deployment results in the creation of a new
bean instance every time a request for that specific bean is made. That is, the bean
is injected into another bean or you request it through a getBean() method call on the
container. As a rule, use the prototype scope for all stateful beans and the singleton
scope for stateless beans.

The following diagram illustrates the Spring prototype scope. A data access object
(DAO) is not typically configured as a prototype, because a typical DAO does not hold
any conversational state; it was just easier for this author to reuse the core of the
singleton diagram.

In contrast to the other scopes, Spring does not manage the complete lifecycle of a
prototype bean: the container instantiates, configures, and otherwise assembles a
prototype object, and hands it to the client, with no further record of that prototype
instance. Thus, although initialization lifecycle callback methods are called on all
objects regardless of scope, in the case of prototypes, configured destruction
lifecycle callbacks are not called. The client code must clean up prototype-scoped
objects and release expensive resources that the prototype bean(s) are holding. To get
the Spring container to release resources held by prototype-scoped beans, try using a
custom bean post-processor, which holds a reference to
beans that need to be cleaned up.

In some respects, the Spring container’s role in regard to a prototype-scoped bean is a
replacement for the Java new operator. All lifecycle management past that point must
be handled by the client. (For details on the lifecycle of a bean in the Spring
container, see Section 6.6.1, “Lifecycle callbacks”.)

6.5.3 Singleton beans with prototype-bean dependencies

When you use singleton-scoped beans with dependencies on prototype beans, be aware that
dependencies are resolved at instantiation time. Thus if you dependency-inject a
prototype-scoped bean into a singleton-scoped bean, a new prototype bean is instantiated
and then dependency-injected into the singleton bean. The prototype instance is the sole
instance that is ever supplied to the singleton-scoped bean.

However, suppose you want the singleton-scoped bean to acquire a new instance of the
prototype-scoped bean repeatedly at runtime. You cannot dependency-inject a
prototype-scoped bean into your singleton bean, because that injection occurs only
once, when the Spring container is instantiating the singleton bean and resolving
and injecting its dependencies. If you need a new instance of a prototype bean at
runtime more than once, see Section 6.4.6, “Method injection”

The request, session, globalSession, application, and websocket scopes are
only available if you use a web-aware Spring ApplicationContext implementation
(such as XmlWebApplicationContext). If you use these scopes with regular Spring IoC
containers such as the ClassPathXmlApplicationContext, an IllegalStateException will
be thrown complaining about an unknown bean scope.

Initial web configuration

To support the scoping of beans at the request, session, globalSession,
application, and websocket levels (web-scoped beans), some minor initial
configuration is required before you define your beans. (This initial setup is not
required for the standard scopes, singleton and prototype.)

How you accomplish this initial setup depends on your particular Servlet environment.

If you access scoped beans within Spring Web MVC, in effect, within a request that is
processed by the Spring DispatcherServlet or DispatcherPortlet, then no special
setup is necessary: DispatcherServlet and DispatcherPortlet already expose all
relevant state.

If you use a Servlet 2.5 web container, with requests processed outside of Spring’s
DispatcherServlet (for example, when using JSF or Struts), you need to register the
org.springframework.web.context.request.RequestContextListenerServletRequestListener.
For Servlet 3.0+, this can be done programmatically via the WebApplicationInitializer
interface. Alternatively, or for older containers, add the following declaration to
your web application’s web.xml file:

Alternatively, if there are issues with your listener setup, consider using Spring’s
RequestContextFilter. The filter mapping depends on the surrounding web
application configuration, so you have to change it as appropriate.

DispatcherServlet, RequestContextListener, and RequestContextFilter all do exactly
the same thing, namely bind the HTTP request object to the Thread that is servicing
that request. This makes beans that are request- and session-scoped available further
down the call chain.

Request scope

Consider the following XML configuration for a bean definition:

<beanid="loginAction"class="com.foo.LoginAction"scope="request"/>

The Spring container creates a new instance of the LoginAction bean by using the
loginAction bean definition for each and every HTTP request. That is, the
loginAction bean is scoped at the HTTP request level. You can change the internal
state of the instance that is created as much as you want, because other instances
created from the same loginAction bean definition will not see these changes in state;
they are particular to an individual request. When the request completes processing, the
bean that is scoped to the request is discarded.

Session scope

The Spring container creates a new instance of the UserPreferences bean by using the
userPreferences bean definition for the lifetime of a single HTTP Session. In other
words, the userPreferences bean is effectively scoped at the HTTP Session level. As
with request-scoped beans, you can change the internal state of the instance that is
created as much as you want, knowing that other HTTP Session instances that are also
using instances created from the same userPreferences bean definition do not see these
changes in state, because they are particular to an individual HTTP Session. When the
HTTP Session is eventually discarded, the bean that is scoped to that particular HTTP
Session is also discarded.

Global session scope

The globalSession scope is similar to the standard HTTP Session scope
(described above), and applies only in the context of
portlet-based web applications. The portlet specification defines the notion of a global
Session that is shared among all portlets that make up a single portlet web
application. Beans defined at the globalSession scope are scoped (or bound) to the
lifetime of the global portlet Session.

If you write a standard Servlet-based web application and you define one or more beans
as having globalSession scope, the standard HTTP Session scope is used, and no
error is raised.

Application scope

The Spring container creates a new instance of the AppPreferences bean by using the
appPreferences bean definition once for the entire web application. That is, the
appPreferences bean is scoped at the ServletContext level, stored as a regular
ServletContext attribute. This is somewhat similar to a Spring singleton bean but
differs in two important ways: It is a singleton per ServletContext, not per Spring
'ApplicationContext' (for which there may be several in any given web application),
and it is actually exposed and therefore visible as a ServletContext attribute.

Scoped beans as dependencies

The Spring IoC container manages not only the instantiation of your objects (beans),
but also the wiring up of collaborators (or dependencies). If you want to inject (for
example) an HTTP request scoped bean into another bean of a longer-lived scope, you may
choose to inject an AOP proxy in place of the scoped bean. That is, you need to inject
a proxy object that exposes the same public interface as the scoped object but that can
also retrieve the real target object from the relevant scope (such as an HTTP request)
and delegate method calls onto the real object.

Note

You may also use <aop:scoped-proxy/> between beans that are scoped as singleton,
with the reference then going through an intermediate proxy that is serializable
and therefore able to re-obtain the target singleton bean on deserialization.

When declaring <aop:scoped-proxy/> against a bean of scope prototype, every method
call on the shared proxy will lead to the creation of a new target instance which the
call is then being forwarded to.

Also, scoped proxies are not the only way to access beans from shorter scopes in a
lifecycle-safe fashion. You may also simply declare your injection point (i.e. the
constructor/setter argument or autowired field) as ObjectFactory<MyTargetBean>,
allowing for a getObject() call to retrieve the current instance on demand every
time it is needed - without holding on to the instance or storing it separately.

The JSR-330 variant of this is called Provider, used with a Provider<MyTargetBean>
declaration and a corresponding get() call for every retrieval attempt.
See here for more details on JSR-330 overall.

The configuration in the following example is only one line, but it is important to
understand the "why" as well as the "how" behind it.

<?xml version="1.0" encoding="UTF-8"?><beansxmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xmlns:aop="http://www.springframework.org/schema/aop"xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/aop
http://www.springframework.org/schema/aop/spring-aop.xsd"><!-- an HTTP Session-scoped bean exposed as a proxy --><beanid="userPreferences"class="com.foo.UserPreferences"scope="session"><!-- instructs the container to proxy the surrounding bean --><aop:scoped-proxy/></bean><!-- a singleton-scoped bean injected with a proxy to the above bean --><beanid="userService"class="com.foo.SimpleUserService"><!-- a reference to the proxied userPreferences bean --><propertyname="userPreferences"ref="userPreferences"/></bean></beans>

To create such a proxy, you insert a child <aop:scoped-proxy/> element into a scoped
bean definition (see the section called “Choosing the type of proxy to create” and
Chapter 40, XML Schema-based configuration). Why do definitions of beans scoped at the request, session,
globalSession and custom-scope levels require the <aop:scoped-proxy/> element?
Let’s examine the following singleton bean definition and contrast it with what you need
to define for the aforementioned scopes (note that the following userPreferences bean
definition as it stands is incomplete).

In the preceding example, the singleton bean userManager is injected with a reference
to the HTTP Session-scoped bean userPreferences. The salient point here is that the
userManager bean is a singleton: it will be instantiated exactly once per
container, and its dependencies (in this case only one, the userPreferences bean) are
also injected only once. This means that the userManager bean will only operate on the
exact same userPreferences object, that is, the one that it was originally injected
with.

This is not the behavior you want when injecting a shorter-lived scoped bean into a
longer-lived scoped bean, for example injecting an HTTP Session-scoped collaborating
bean as a dependency into singleton bean. Rather, you need a single userManager
object, and for the lifetime of an HTTP Session, you need a userPreferences object
that is specific to said HTTP Session. Thus the container creates an object that
exposes the exact same public interface as the UserPreferences class (ideally an
object that is aUserPreferences instance) which can fetch the real
UserPreferences object from the scoping mechanism (HTTP request, Session, etc.). The
container injects this proxy object into the userManager bean, which is unaware that
this UserPreferences reference is a proxy. In this example, when a UserManager
instance invokes a method on the dependency-injected UserPreferences object, it
actually is invoking a method on the proxy. The proxy then fetches the real
UserPreferences object from (in this case) the HTTP Session, and delegates the
method invocation onto the retrieved real UserPreferences object.

Thus you need the following, correct and complete, configuration when injecting
request-, session-, and globalSession-scoped beans into collaborating objects:

Choosing the type of proxy to create

By default, when the Spring container creates a proxy for a bean that is marked up with
the <aop:scoped-proxy/> element, a CGLIB-based class proxy is created.

Note

CGLIB proxies only intercept public method calls! Do not call non-public methods
on such a proxy; they will not be delegated to the actual scoped target object.

Alternatively, you can configure the Spring container to create standard JDK
interface-based proxies for such scoped beans, by specifying false for the value of
the proxy-target-class attribute of the <aop:scoped-proxy/> element. Using JDK
interface-based proxies means that you do not need additional libraries in your
application classpath to effect such proxying. However, it also means that the class of
the scoped bean must implement at least one interface, and that all collaborators
into which the scoped bean is injected must reference the bean through one of its
interfaces.

6.5.5 Custom scopes

The bean scoping mechanism is extensible; You can define your own
scopes, or even redefine existing scopes, although the latter is considered bad practice
and you cannot override the built-in singleton and prototype scopes.

Creating a custom scope

To integrate your custom scope(s) into the Spring container, you need to implement the
org.springframework.beans.factory.config.Scope interface, which is described in this
section. For an idea of how to implement your own scopes, see the Scope
implementations that are supplied with the Spring Framework itself and the
Scope javadocs,
which explains the methods you need to implement in more detail.

The Scope interface has four methods to get objects from the scope, remove them from
the scope, and allow them to be destroyed.

The following method returns the object from the underlying scope. The session scope
implementation, for example, returns the session-scoped bean (and if it does not exist,
the method returns a new instance of the bean, after having bound it to the session for
future reference).

Object get(String name, ObjectFactory objectFactory)

The following method removes the object from the underlying scope. The session scope
implementation for example, removes the session-scoped bean from the underlying session.
The object should be returned, but you can return null if the object with the specified
name is not found.

Object remove(String name)

The following method registers the callbacks the scope should execute when it is
destroyed or when the specified object in the scope is destroyed. Refer to the javadocs
or a Spring scope implementation for more information on destruction callbacks.

The following method obtains the conversation identifier for the underlying scope. This
identifier is different for each scope. For a session scoped implementation, this
identifier can be the session identifier.

String getConversationId()

Using a custom scope

After you write and test one or more custom Scope implementations, you need to make
the Spring container aware of your new scope(s). The following method is the central
method to register a new Scope with the Spring container:

void registerScope(String scopeName, Scope scope);

This method is declared on the ConfigurableBeanFactory interface, which is available
on most of the concrete ApplicationContext implementations that ship with Spring via
the BeanFactory property.

The first argument to the registerScope(..) method is the unique name associated with
a scope; examples of such names in the Spring container itself are singleton and
prototype. The second argument to the registerScope(..) method is an actual instance
of the custom Scope implementation that you wish to register and use.

Suppose that you write your custom Scope implementation, and then register it as below.

Note

The example below uses SimpleThreadScope which is included with Spring, but not
registered by default. The instructions would be the same for your own custom Scope
implementations.

When you place <aop:scoped-proxy/> in a FactoryBean implementation, it is the factory
bean itself that is scoped, not the object returned from getObject().

6.6 Customizing the nature of a bean

6.6.1 Lifecycle callbacks

To interact with the container’s management of the bean lifecycle, you can implement the
Spring InitializingBean and DisposableBean interfaces. The container calls
afterPropertiesSet() for the former and destroy() for the latter to allow the bean
to perform certain actions upon initialization and destruction of your beans.

Tip

The JSR-250 @PostConstruct and @PreDestroy annotations are generally considered best
practice for receiving lifecycle callbacks in a modern Spring application. Using these
annotations means that your beans are not coupled to Spring specific interfaces. For
details see Section 6.9.8, “@PostConstruct and @PreDestroy”.

If you don’t want to use the JSR-250 annotations but you are still looking to remove
coupling consider the use of init-method and destroy-method object definition metadata.

Internally, the Spring Framework uses BeanPostProcessor implementations to process any
callback interfaces it can find and call the appropriate methods. If you need custom
features or other lifecycle behavior Spring does not offer out-of-the-box, you can
implement a BeanPostProcessor yourself. For more information, see
Section 6.8, “Container Extension Points”.

In addition to the initialization and destruction callbacks, Spring-managed objects may
also implement the Lifecycle interface so that those objects can participate in the
startup and shutdown process as driven by the container’s own lifecycle.

The lifecycle callback interfaces are described in this section.

Initialization callbacks

The org.springframework.beans.factory.InitializingBean interface allows a bean to
perform initialization work after all necessary properties on the bean have been set by
the container. The InitializingBean interface specifies a single method:

void afterPropertiesSet() throws Exception;

It is recommended that you do not use the InitializingBean interface because it
unnecessarily couples the code to Spring. Alternatively, use
the @PostConstruct annotation or
specify a POJO initialization method. In the case of XML-based configuration metadata,
you use the init-method attribute to specify the name of the method that has a void
no-argument signature. With Java config, you use the initMethod attribute of @Bean,
see the section called “Receiving lifecycle callbacks”. For example, the following:

Destruction callbacks

Implementing the org.springframework.beans.factory.DisposableBean interface allows a
bean to get a callback when the container containing it is destroyed. The
DisposableBean interface specifies a single method:

void destroy() throws Exception;

It is recommended that you do not use the DisposableBean callback interface because it
unnecessarily couples the code to Spring. Alternatively, use
the @PreDestroy annotation or
specify a generic method that is supported by bean definitions. With XML-based
configuration metadata, you use the destroy-method attribute on the <bean/>.
With Java config, you use the destroyMethod attribute of @Bean, see
the section called “Receiving lifecycle callbacks”. For example, the following definition:

The destroy-method attribute of a <bean> element can be assigned a special
(inferred) value which instructs Spring to automatically detect a public close or
shutdown method on the specific bean class (any class that implements
java.lang.AutoCloseable or java.io.Closeable would therefore match). This special
(inferred) value can also be set on the default-destroy-method attribute of a
<beans> element to apply this behavior to an entire set of beans (see
the section called “Default initialization and destroy methods”). Note that this is the
default behavior with Java config.

Default initialization and destroy methods

When you write initialization and destroy method callbacks that do not use the
Spring-specific InitializingBean and DisposableBean callback interfaces, you
typically write methods with names such as init(), initialize(), dispose(), and so
on. Ideally, the names of such lifecycle callback methods are standardized across a
project so that all developers use the same method names and ensure consistency.

You can configure the Spring container to look for named initialization and destroy
callback method names on every bean. This means that you, as an application
developer, can write your application classes and use an initialization callback called
init(), without having to configure an init-method="init" attribute with each bean
definition. The Spring IoC container calls that method when the bean is created (and in
accordance with the standard lifecycle callback contract described previously). This
feature also enforces a consistent naming convention for initialization and destroy
method callbacks.

Suppose that your initialization callback methods are named init() and destroy
callback methods are named destroy(). Your class will resemble the class in the
following example.

The presence of the default-init-method attribute on the top-level <beans/> element
attribute causes the Spring IoC container to recognize a method called init on beans
as the initialization method callback. When a bean is created and assembled, if the bean
class has such a method, it is invoked at the appropriate time.

You configure destroy method callbacks similarly (in XML, that is) by using the
default-destroy-method attribute on the top-level <beans/> element.

Where existing bean classes already have callback methods that are named at variance
with the convention, you can override the default by specifying (in XML, that is) the
method name using the init-method and destroy-method attributes of the <bean/>
itself.

The Spring container guarantees that a configured initialization callback is called
immediately after a bean is supplied with all dependencies. Thus the initialization
callback is called on the raw bean reference, which means that AOP interceptors and so
forth are not yet applied to the bean. A target bean is fully created first,
then an AOP proxy (for example) with its interceptor chain is applied. If the target
bean and the proxy are defined separately, your code can even interact with the raw
target bean, bypassing the proxy. Hence, it would be inconsistent to apply the
interceptors to the init method, because doing so would couple the lifecycle of the
target bean with its proxy/interceptors and leave strange semantics when your code
interacts directly to the raw target bean.

If multiple lifecycle mechanisms are configured for a bean, and each mechanism is
configured with a different method name, then each configured method is executed in the
order listed below. However, if the same method name is configured - for example,
init() for an initialization method - for more than one of these lifecycle mechanisms,
that method is executed once, as explained in the preceding section.

Multiple lifecycle mechanisms configured for the same bean, with different
initialization methods, are called as follows:

Methods annotated with @PostConstruct

afterPropertiesSet() as defined by the InitializingBean callback interface

A custom configured init() method

Destroy methods are called in the same order:

Methods annotated with @PreDestroy

destroy() as defined by the DisposableBean callback interface

A custom configured destroy() method

Startup and shutdown callbacks

The Lifecycle interface defines the essential methods for any object that has its own
lifecycle requirements (e.g. starts and stops some background process):

Any Spring-managed object may implement that interface. Then, when the
ApplicationContext itself receives start and stop signals, e.g. for a stop/restart
scenario at runtime, it will cascade those calls to all Lifecycle implementations
defined within that context. It does this by delegating to a LifecycleProcessor:

Notice that the LifecycleProcessor is itself an extension of the Lifecycle
interface. It also adds two other methods for reacting to the context being refreshed
and closed.

Tip

Note that the regular org.springframework.context.Lifecycle interface is just a plain
contract for explicit start/stop notifications and does NOT imply auto-startup at context
refresh time. Consider implementing org.springframework.context.SmartLifecycle instead
for fine-grained control over auto-startup of a specific bean (including startup phases).
Also, please note that stop notifications are not guaranteed to come before destruction:
On regular shutdown, all Lifecycle beans will first receive a stop notification before
the general destruction callbacks are being propagated; however, on hot refresh during a
context’s lifetime or on aborted refresh attempts, only destroy methods will be called.

The order of startup and shutdown invocations can be important. If a "depends-on"
relationship exists between any two objects, the dependent side will start after its
dependency, and it will stop before its dependency. However, at times the direct
dependencies are unknown. You may only know that objects of a certain type should start
prior to objects of another type. In those cases, the SmartLifecycle interface defines
another option, namely the getPhase() method as defined on its super-interface,
Phased.

When starting, the objects with the lowest phase start first, and when stopping, the
reverse order is followed. Therefore, an object that implements SmartLifecycle and
whose getPhase() method returns Integer.MIN_VALUE would be among the first to start
and the last to stop. At the other end of the spectrum, a phase value of
Integer.MAX_VALUE would indicate that the object should be started last and stopped
first (likely because it depends on other processes to be running). When considering the
phase value, it’s also important to know that the default phase for any "normal"
Lifecycle object that does not implement SmartLifecycle would be 0. Therefore, any
negative phase value would indicate that an object should start before those standard
components (and stop after them), and vice versa for any positive phase value.

As you can see the stop method defined by SmartLifecycle accepts a callback. Any
implementation must invoke that callback’s run() method after that implementation’s
shutdown process is complete. That enables asynchronous shutdown where necessary since
the default implementation of the LifecycleProcessor interface,
DefaultLifecycleProcessor, will wait up to its timeout value for the group of objects
within each phase to invoke that callback. The default per-phase timeout is 30 seconds.
You can override the default lifecycle processor instance by defining a bean named
"lifecycleProcessor" within the context. If you only want to modify the timeout, then
defining the following would be sufficient:

As mentioned, the LifecycleProcessor interface defines callback methods for the
refreshing and closing of the context as well. The latter will simply drive the shutdown
process as if stop() had been called explicitly, but it will happen when the context is
closing. The 'refresh' callback on the other hand enables another feature of
SmartLifecycle beans. When the context is refreshed (after all objects have been
instantiated and initialized), that callback will be invoked, and at that point the
default lifecycle processor will check the boolean value returned by each
SmartLifecycle object’s isAutoStartup() method. If "true", then that object will be
started at that point rather than waiting for an explicit invocation of the context’s or
its own start() method (unlike the context refresh, the context start does not happen
automatically for a standard context implementation). The "phase" value as well as any
"depends-on" relationships will determine the startup order in the same way as described
above.

Shutting down the Spring IoC container gracefully in non-web applications

Note

This section applies only to non-web applications. Spring’s web-based
ApplicationContext implementations already have code in place to shut down the Spring
IoC container gracefully when the relevant web application is shut down.

If you are using Spring’s IoC container in a non-web application environment; for
example, in a rich client desktop environment; you register a shutdown hook with the
JVM. Doing so ensures a graceful shutdown and calls the relevant destroy methods on your
singleton beans so that all resources are released. Of course, you must still configure
and implement these destroy callbacks correctly.

To register a shutdown hook, you call the registerShutdownHook() method that is
declared on the ConfigurableApplicationContext interface:

6.6.2 ApplicationContextAware and BeanNameAware

When an ApplicationContext creates an object instance that implements the
org.springframework.context.ApplicationContextAware interface, the instance is provided
with a reference to that ApplicationContext.

Thus beans can manipulate programmatically the ApplicationContext that created them,
through the ApplicationContext interface, or by casting the reference to a known
subclass of this interface, such as ConfigurableApplicationContext, which exposes
additional functionality. One use would be the programmatic retrieval of other beans.
Sometimes this capability is useful; however, in general you should avoid it, because it
couples the code to Spring and does not follow the Inversion of Control style, where
collaborators are provided to beans as properties. Other methods of the
ApplicationContext provide access to file resources, publishing application events, and
accessing a MessageSource. These additional features are described in
Section 6.15, “Additional Capabilities of the ApplicationContext”

As of Spring 2.5, autowiring is another alternative to obtain reference to the
ApplicationContext. The "traditional" constructor and byType autowiring modes (as
described in Section 6.4.5, “Autowiring collaborators”) can provide a dependency of type
ApplicationContext for a constructor argument or setter method parameter,
respectively. For more flexibility, including the ability to autowire fields and
multiple parameter methods, use the new annotation-based autowiring features. If you do,
the ApplicationContext is autowired into a field, constructor argument, or method
parameter that is expecting the ApplicationContext type if the field, constructor, or
method in question carries the @Autowired annotation. For more information, see
Section 6.9.2, “@Autowired”.

When an ApplicationContext creates a class that implements the
org.springframework.beans.factory.BeanNameAware interface, the class is provided with
a reference to the name defined in its associated object definition.

The callback is invoked after population of normal bean properties but before an
initialization callback such as InitializingBeanafterPropertiesSet or a custom
init-method.

6.6.3 Other Aware interfaces

Besides ApplicationContextAware and BeanNameAware discussed above, Spring offers a
range of Aware interfaces that allow beans to indicate to the container that they
require a certain infrastructure dependency. The most important Aware interfaces
are summarized below - as a general rule, the name is a good indication of the
dependency type:

Note again that usage of these interfaces ties your code to the Spring API and does not
follow the Inversion of Control style. As such, they are recommended for infrastructure
beans that require programmatic access to the container.

6.7 Bean definition inheritance

A bean definition can contain a lot of configuration information, including constructor
arguments, property values, and container-specific information such as initialization
method, static factory method name, and so on. A child bean definition inherits
configuration data from a parent definition. The child definition can override some
values, or add others, as needed. Using parent and child bean definitions can save a lot
of typing. Effectively, this is a form of templating.

If you work with an ApplicationContext interface programmatically, child bean
definitions are represented by the ChildBeanDefinition class. Most users do not work
with them on this level, instead configuring bean definitions declaratively in something
like the ClassPathXmlApplicationContext. When you use XML-based configuration
metadata, you indicate a child bean definition by using the parent attribute,
specifying the parent bean as the value of this attribute.

<beanid="inheritedTestBean"abstract="true"class="org.springframework.beans.TestBean"><propertyname="name"value="parent"/><propertyname="age"value="1"/></bean><beanid="inheritsWithDifferentClass"class="org.springframework.beans.DerivedTestBean"parent="inheritedTestBean" init-method="initialize">
<propertyname="name"value="override"/><!-- the age property value of 1 will be inherited from parent --></bean>

A child bean definition uses the bean class from the parent definition if none is
specified, but can also override it. In the latter case, the child bean class must be
compatible with the parent, that is, it must accept the parent’s property values.

The preceding example explicitly marks the parent bean definition as abstract by using
the abstract attribute. If the parent definition does not specify a class, explicitly
marking the parent bean definition as abstract is required, as follows:

<beanid="inheritedTestBeanWithoutClass"abstract="true"><propertyname="name"value="parent"/><propertyname="age"value="1"/></bean><beanid="inheritsWithClass"class="org.springframework.beans.DerivedTestBean"parent="inheritedTestBeanWithoutClass"init-method="initialize"><propertyname="name"value="override"/><!-- age will inherit the value of 1 from the parent bean definition--></bean>

The parent bean cannot be instantiated on its own because it is incomplete, and it is
also explicitly marked as abstract. When a definition is abstract like this, it is
usable only as a pure template bean definition that serves as a parent definition for
child definitions. Trying to use such an abstract parent bean on its own, by referring
to it as a ref property of another bean or doing an explicit getBean() call with the
parent bean id, returns an error. Similarly, the container’s internal
preInstantiateSingletons() method ignores bean definitions that are defined as
abstract.

Note

ApplicationContext pre-instantiates all singletons by default. Therefore, it is
important (at least for singleton beans) that if you have a (parent) bean definition
which you intend to use only as a template, and this definition specifies a class, you
must make sure to set the abstract attribute to true, otherwise the application
context will actually (attempt to) pre-instantiate the abstract bean.

6.8 Container Extension Points

Typically, an application developer does not need to subclass ApplicationContext
implementation classes. Instead, the Spring IoC container can be extended by plugging in
implementations of special integration interfaces. The next few sections describe these
integration interfaces.

6.8.1 Customizing beans using a BeanPostProcessor

The BeanPostProcessor interface defines callback methods that you can implement to
provide your own (or override the container’s default) instantiation logic,
dependency-resolution logic, and so forth. If you want to implement some custom logic
after the Spring container finishes instantiating, configuring, and initializing a bean,
you can plug in one or more BeanPostProcessor implementations.

You can configure multiple BeanPostProcessor instances, and you can control the order
in which these BeanPostProcessors execute by setting the order property. You can
set this property only if the BeanPostProcessor implements the Ordered interface; if
you write your own BeanPostProcessor you should consider implementing the Ordered
interface too. For further details, consult the javadocs of the BeanPostProcessor and
Ordered interfaces. See also the note below on
programmatic
registration of BeanPostProcessors.

Note

BeanPostProcessors operate on bean (or object) instances; that is to say, the
Spring IoC container instantiates a bean instance and then BeanPostProcessors do
their work.

BeanPostProcessors are scoped per-container. This is only relevant if you are
using container hierarchies. If you define a BeanPostProcessor in one container, it
will only post-process the beans in that container. In other words, beans that are
defined in one container are not post-processed by a BeanPostProcessor defined in
another container, even if both containers are part of the same hierarchy.

The org.springframework.beans.factory.config.BeanPostProcessor interface consists of
exactly two callback methods. When such a class is registered as a post-processor with
the container, for each bean instance that is created by the container, the
post-processor gets a callback from the container both before container
initialization methods (such as InitializingBean’s afterPropertiesSet() and any
declared init method) are called as well as after any bean initialization callbacks.
The post-processor can take any action with the bean instance, including ignoring the
callback completely. A bean post-processor typically checks for callback interfaces or
may wrap a bean with a proxy. Some Spring AOP infrastructure classes are implemented as
bean post-processors in order to provide proxy-wrapping logic.

An ApplicationContextautomatically detects any beans that are defined in the
configuration metadata which implement the BeanPostProcessor interface. The
ApplicationContext registers these beans as post-processors so that they can be called
later upon bean creation. Bean post-processors can be deployed in the container just
like any other beans.

Note that when declaring a BeanPostProcessor using an @Bean factory method on a
configuration class, the return type of the factory method should be the implementation
class itself or at least the org.springframework.beans.factory.config.BeanPostProcessor
interface, clearly indicating the post-processor nature of that bean. Otherwise, the
ApplicationContext won’t be able to autodetect it by type before fully creating it.
Since a BeanPostProcessor needs to be instantiated early in order to apply to the
initialization of other beans in the context, this early type detection is critical.

Programmatically registering BeanPostProcessors

While the recommended approach for BeanPostProcessor registration is through
ApplicationContext auto-detection (as described above), it is also possible to
register them programmatically against a ConfigurableBeanFactory using the
addBeanPostProcessor method. This can be useful when needing to evaluate conditional
logic before registration, or even for copying bean post processors across contexts in a
hierarchy. Note however that BeanPostProcessors added programmatically do not
respect the Ordered interface. Here it is the order of registration that
dictates the order of execution. Note also that BeanPostProcessors registered
programmatically are always processed before those registered through auto-detection,
regardless of any explicit ordering.

BeanPostProcessors and AOP auto-proxying

Classes that implement the BeanPostProcessor interface are special and are treated
differently by the container. All BeanPostProcessorsand beans that they reference
directly are instantiated on startup, as part of the special startup phase of the
ApplicationContext. Next, all BeanPostProcessors are registered in a sorted fashion
and applied to all further beans in the container. Because AOP auto-proxying is
implemented as a BeanPostProcessor itself, neither BeanPostProcessors nor the beans
they reference directly are eligible for auto-proxying, and thus do not have aspects
woven into them.

For any such bean, you should see an informational log message: "Bean foo is not
eligible for getting processed by all BeanPostProcessor interfaces (for example: not
eligible for auto-proxying)".

Note that if you have beans wired into your BeanPostProcessor using autowiring or
@Resource (which may fall back to autowiring), Spring might access unexpected beans
when searching for type-matching dependency candidates, and therefore make them
ineligible for auto-proxying or other kinds of bean post-processing. For example, if you
have a dependency annotated with @Resource where the field/setter name does not
directly correspond to the declared name of a bean and no name attribute is used, then
Spring will access other beans for matching them by type.

The following examples show how to write, register, and use BeanPostProcessors in an
ApplicationContext.

Example: Hello World, BeanPostProcessor-style

This first example illustrates basic usage. The example shows a custom
BeanPostProcessor implementation that invokes the toString() method of each bean as
it is created by the container and prints the resulting string to the system console.

<?xml version="1.0" encoding="UTF-8"?><beansxmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xmlns:lang="http://www.springframework.org/schema/lang"xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/lang
http://www.springframework.org/schema/lang/spring-lang.xsd"><lang:groovyid="messenger"script-source="classpath:org/springframework/scripting/groovy/Messenger.groovy"><lang:propertyname="message"value="Fiona Apple Is Just So Dreamy."/></lang:groovy><!--
when the above bean (messenger) is instantiated, this custom
BeanPostProcessor implementation will output the fact to the system console
--><beanclass="scripting.InstantiationTracingBeanPostProcessor"/></beans>

Notice how the InstantiationTracingBeanPostProcessor is simply defined. It does not
even have a name, and because it is a bean it can be dependency-injected just like any
other bean. (The preceding configuration also defines a bean that is backed by a Groovy
script. The Spring dynamic language support is detailed in the chapter entitled
Chapter 34, Dynamic language support.)

The following simple Java application executes the preceding code and configuration:

Example: The RequiredAnnotationBeanPostProcessor

Using callback interfaces or annotations in conjunction with a custom
BeanPostProcessor implementation is a common means of extending the Spring IoC
container. An example is Spring’s RequiredAnnotationBeanPostProcessor - a
BeanPostProcessor implementation that ships with the Spring distribution which ensures
that JavaBean properties on beans that are marked with an (arbitrary) annotation are
actually (configured to be) dependency-injected with a value.

The next extension point that we will look at is the
org.springframework.beans.factory.config.BeanFactoryPostProcessor. The semantics of
this interface are similar to those of the BeanPostProcessor, with one major
difference: BeanFactoryPostProcessor operates on the bean configuration metadata;
that is, the Spring IoC container allows a BeanFactoryPostProcessor to read the
configuration metadata and potentially change it before the container instantiates
any beans other than BeanFactoryPostProcessors.

You can configure multiple BeanFactoryPostProcessors, and you can control the order in
which these BeanFactoryPostProcessors execute by setting the order property.
However, you can only set this property if the BeanFactoryPostProcessor implements the
Ordered interface. If you write your own BeanFactoryPostProcessor, you should
consider implementing the Ordered interface too. Consult the javadocs of the
BeanFactoryPostProcessor and Ordered interfaces for more details.

Note

If you want to change the actual bean instances (i.e., the objects that are created
from the configuration metadata), then you instead need to use a BeanPostProcessor
(described above in Section 6.8.1, “Customizing beans using a BeanPostProcessor”). While it is technically possible
to work with bean instances within a BeanFactoryPostProcessor (e.g., using
BeanFactory.getBean()), doing so causes premature bean instantiation, violating the
standard container lifecycle. This may cause negative side effects such as bypassing
bean post processing.

Also, BeanFactoryPostProcessors are scoped per-container. This is only relevant if
you are using container hierarchies. If you define a BeanFactoryPostProcessor in one
container, it will only be applied to the bean definitions in that container. Bean
definitions in one container will not be post-processed by BeanFactoryPostProcessors
in another container, even if both containers are part of the same hierarchy.

A bean factory post-processor is executed automatically when it is declared inside an
ApplicationContext, in order to apply changes to the configuration metadata that
define the container. Spring includes a number of predefined bean factory
post-processors, such as PropertyOverrideConfigurer and
PropertyPlaceholderConfigurer. A custom BeanFactoryPostProcessor can also be used,
for example, to register custom property editors.

An ApplicationContext automatically detects any beans that are deployed into it that
implement the BeanFactoryPostProcessor interface. It uses these beans as bean factory
post-processors, at the appropriate time. You can deploy these post-processor beans as
you would any other bean.

Note

As with BeanPostProcessors , you typically do not want to configure
BeanFactoryPostProcessors for lazy initialization. If no other bean references a
Bean(Factory)PostProcessor, that post-processor will not get instantiated at all.
Thus, marking it for lazy initialization will be ignored, and the
Bean(Factory)PostProcessor will be instantiated eagerly even if you set the
default-lazy-init attribute to true on the declaration of your <beans /> element.

Example: the Class name substitution PropertyPlaceholderConfigurer

You use the PropertyPlaceholderConfigurer to externalize property values from a bean
definition in a separate file using the standard Java Properties format. Doing so
enables the person deploying an application to customize environment-specific properties
such as database URLs and passwords, without the complexity or risk of modifying the
main XML definition file or files for the container.

Consider the following XML-based configuration metadata fragment, where a DataSource
with placeholder values is defined. The example shows properties configured from an
external Properties file. At runtime, a PropertyPlaceholderConfigurer is applied to
the metadata that will replace some properties of the DataSource. The values to replace
are specified as placeholders of the form ${property-name} which follows the Ant /
log4j / JSP EL style.

Therefore, the string ${jdbc.username} is replaced at runtime with the value 'sa', and
the same applies for other placeholder values that match keys in the properties file.
The PropertyPlaceholderConfigurer checks for placeholders in most properties and
attributes of a bean definition. Furthermore, the placeholder prefix and suffix can be
customized.

With the context namespace introduced in Spring 2.5, it is possible to configure
property placeholders with a dedicated configuration element. One or more locations can
be provided as a comma-separated list in the location attribute.

The PropertyPlaceholderConfigurer not only looks for properties in the Properties
file you specify. By default it also checks against the Java System properties if it
cannot find a property in the specified properties files. You can customize this
behavior by setting the systemPropertiesMode property of the configurer with one of
the following three supported integer values:

never (0): Never check system properties

fallback (1): Check system properties if not resolvable in the specified
properties files. This is the default.

override (2): Check system properties first, before trying the specified
properties files. This allows system properties to override any other property source.

Consult the PropertyPlaceholderConfigurer javadocs for more information.

Tip

You can use the PropertyPlaceholderConfigurer to substitute class names, which is
sometimes useful when you have to pick a particular implementation class at runtime. For
example:

If the class cannot be resolved at runtime to a valid class, resolution of the bean
fails when it is about to be created, which is during the preInstantiateSingletons()
phase of an ApplicationContext for a non-lazy-init bean.

Example: the PropertyOverrideConfigurer

The PropertyOverrideConfigurer, another bean factory post-processor, resembles the
PropertyPlaceholderConfigurer, but unlike the latter, the original definitions can
have default values or no values at all for bean properties. If an overriding
Properties file does not have an entry for a certain bean property, the default
context definition is used.

Note that the bean definition is not aware of being overridden, so it is not
immediately obvious from the XML definition file that the override configurer is being
used. In case of multiple PropertyOverrideConfigurer instances that define different
values for the same bean property, the last one wins, due to the overriding mechanism.

This example file can be used with a container definition that contains a bean called
dataSource, which has driver and url properties.

Compound property names are also supported, as long as every component of the path
except the final property being overridden is already non-null (presumably initialized
by the constructors). In this example…​

foo.fred.bob.sammy=123

the sammy property of the bob property of the fred property of the foo bean
is set to the scalar value 123.

Note

Specified override values are always literal values; they are not translated into
bean references. This convention also applies when the original value in the XML bean
definition specifies a bean reference.

With the context namespace introduced in Spring 2.5, it is possible to configure
property overriding with a dedicated configuration element:

<context:property-overridelocation="classpath:override.properties"/>

6.8.3 Customizing instantiation logic with a FactoryBean

Implement the org.springframework.beans.factory.FactoryBean interface for objects that
are themselves factories.

The FactoryBean interface is a point of pluggability into the Spring IoC container’s
instantiation logic. If you have complex initialization code that is better expressed in
Java as opposed to a (potentially) verbose amount of XML, you can create your own
FactoryBean, write the complex initialization inside that class, and then plug your
custom FactoryBean into the container.

The FactoryBean interface provides three methods:

Object getObject(): returns an instance of the object this factory creates. The
instance can possibly be shared, depending on whether this factory returns singletons
or prototypes.

Class getObjectType(): returns the object type returned by the getObject() method
or null if the type is not known in advance.

The FactoryBean concept and interface is used in a number of places within the Spring
Framework; more than 50 implementations of the FactoryBean interface ship with Spring
itself.

When you need to ask a container for an actual FactoryBean instance itself instead of
the bean it produces, preface the bean’s id with the ampersand symbol ( &) when
calling the getBean() method of the ApplicationContext. So for a given FactoryBean
with an id of myBean, invoking getBean("myBean") on the container returns the
product of the FactoryBean; whereas, invoking getBean("&myBean") returns the
FactoryBean instance itself.

6.9 Annotation-based container configuration

Are annotations better than XML for configuring Spring?

The introduction of annotation-based configurations raised the question of whether this
approach is 'better' than XML. The short answer is it depends. The long answer is
that each approach has its pros and cons, and usually it is up to the developer to
decide which strategy suits them better. Due to the way they are defined, annotations
provide a lot of context in their declaration, leading to shorter and more concise
configuration. However, XML excels at wiring up components without touching their source
code or recompiling them. Some developers prefer having the wiring close to the source
while others argue that annotated classes are no longer POJOs and, furthermore, that the
configuration becomes decentralized and harder to control.

No matter the choice, Spring can accommodate both styles and even mix them together.
It’s worth pointing out that through its JavaConfig option, Spring allows
annotations to be used in a non-invasive way, without touching the target components
source code and that in terms of tooling, all configuration styles are supported by the
Spring Tool Suite.

An alternative to XML setups is provided by annotation-based configuration which rely on
the bytecode metadata for wiring up components instead of angle-bracket declarations.
Instead of using XML to describe a bean wiring, the developer moves the configuration
into the component class itself by using annotations on the relevant class, method, or
field declaration. As mentioned in the section called “Example: The RequiredAnnotationBeanPostProcessor”, using
a BeanPostProcessor in conjunction with annotations is a common means of extending the
Spring IoC container. For example, Spring 2.0 introduced the possibility of enforcing
required properties with the @Required annotation. Spring
2.5 made it possible to follow that same general approach to drive Spring’s dependency
injection. Essentially, the @Autowired annotation provides the same capabilities as
described in Section 6.4.5, “Autowiring collaborators” but with more fine-grained control and wider
applicability. Spring 2.5 also added support for JSR-250 annotations such as
@PostConstruct, and @PreDestroy. Spring 3.0 added support for JSR-330 (Dependency
Injection for Java) annotations contained in the javax.inject package such as @Inject
and @Named. Details about those annotations can be found in the
relevant section.

Note

Annotation injection is performed before XML injection, thus the latter
configuration will override the former for properties wired through both approaches.

As always, you can register them as individual bean definitions, but they can also be
implicitly registered by including the following tag in an XML-based Spring
configuration (notice the inclusion of the context namespace):

<context:annotation-config/> only looks for annotations on beans in the same
application context in which it is defined. This means that, if you put
<context:annotation-config/> in a WebApplicationContext for a DispatcherServlet,
it only checks for @Autowired beans in your controllers, and not your services. See
Section 21.2, “The DispatcherServlet” for more information.

6.9.1 @Required

The @Required annotation applies to bean property setter methods, as in the following
example:

This annotation simply indicates that the affected bean property must be populated at
configuration time, through an explicit property value in a bean definition or through
autowiring. The container throws an exception if the affected bean property has not been
populated; this allows for eager and explicit failure, avoiding NullPointerExceptions
or the like later on. It is still recommended that you put assertions into the bean
class itself, for example, into an init method. Doing so enforces those required
references and values even when you use the class outside of a container.

6.9.2 @Autowired

As expected, you can apply the @Autowired annotation to "traditional" setter methods:

Your beans can implement the org.springframework.core.Ordered interface or either use
the @Order or standard @Priority annotation if you want items in the array or list
to be sorted into a specific order.

Even typed Maps can be autowired as long as the expected key type is String. The Map
values will contain all beans of the expected type, and the keys will contain the
corresponding bean names:

By default, the autowiring fails whenever zero candidate beans are available; the
default behavior is to treat annotated methods, constructors, and fields as
indicating required dependencies. This behavior can be changed as demonstrated below.

Only one annotated constructor per-class can be marked as required, but multiple
non-required constructors can be annotated. In that case, each is considered among the
candidates and Spring uses the greediest constructor whose dependencies can be
satisfied, that is the constructor that has the largest number of arguments.

@Autowired’s required attribute is recommended over the [email protected] annotation.
The required attribute indicates that the property is not required for autowiring
purposes, the property is ignored if it cannot be autowired. @Required, on the other
hand, is stronger in that it enforces the property that was set by any means supported
by the container. If no value is injected, a corresponding exception is raised.

You can also use @Autowired for interfaces that are well-known resolvable
dependencies: BeanFactory, ApplicationContext, Environment, ResourceLoader,
ApplicationEventPublisher, and MessageSource. These interfaces and their extended
interfaces, such as ConfigurableApplicationContext or ResourcePatternResolver, are
automatically resolved, with no special setup necessary.

@Autowired, @Inject, @Resource, and @Value annotations are handled by Spring
BeanPostProcessor implementations which in turn means that you cannot apply these
annotations within your own BeanPostProcessor or BeanFactoryPostProcessor types (if
any). These types must be 'wired up' explicitly via XML or using a Spring @Bean method.

6.9.3 Fine-tuning annotation-based autowiring with @Primary

Because autowiring by type may lead to multiple candidates, it is often necessary to have
more control over the selection process. One way to accomplish this is with Spring’s
@Primary annotation. @Primary indicates that a particular bean should be given
preference when multiple beans are candidates to be autowired to a single-valued
dependency. If exactly one 'primary' bean exists among the candidates, it will be the
autowired value.

Let’s assume we have the following configuration that defines firstMovieCatalog as the
primaryMovieCatalog.

6.9.4 Fine-tuning annotation-based autowiring with qualifiers

@Primary is an effective way to use autowiring by type with several instances when one
primary candidate can be determined. When more control over the selection process is
required, Spring’s @Qualifier annotation can be used. You can associate qualifier values
with specific arguments, narrowing the set of type matches so that a specific bean is
chosen for each argument. In the simplest case, this can be a plain descriptive value:

For a fallback match, the bean name is considered a default qualifier value. Thus you
can define the bean with an id "main" instead of the nested qualifier element, leading
to the same matching result. However, although you can use this convention to refer to
specific beans by name, @Autowired is fundamentally about type-driven injection with
optional semantic qualifiers. This means that qualifier values, even with the bean name
fallback, always have narrowing semantics within the set of type matches; they do not
semantically express a reference to a unique bean id. Good qualifier values are "main"
or "EMEA" or "persistent", expressing characteristics of a specific component that are
independent from the bean id, which may be auto-generated in case of an anonymous bean
definition like the one in the preceding example.

Qualifiers also apply to typed collections, as discussed above, for example, to
Set<MovieCatalog>. In this case, all matching beans according to the declared
qualifiers are injected as a collection. This implies that qualifiers do not have to be
unique; they rather simply constitute filtering criteria. For example, you can define
multiple MovieCatalog beans with the same qualifier value "action", all of which would
be injected into a Set<MovieCatalog> annotated with @Qualifier("action").

Tip

If you intend to express annotation-driven injection by name, do not primarily use
@Autowired, even if is technically capable of referring to a bean name through
@Qualifier values. Instead, use the JSR-250 @Resource annotation, which is
semantically defined to identify a specific target component by its unique name, with
the declared type being irrelevant for the matching process.

As a specific consequence of this semantic difference, beans that are themselves defined
as a collection or map type cannot be injected through @Autowired, because type
matching is not properly applicable to them. Use @Resource for such beans, referring
to the specific collection or map bean by unique name.

@Autowired applies to fields, constructors, and multi-argument methods, allowing for
narrowing through qualifier annotations at the parameter level. By contrast, @Resource
is supported only for fields and bean property setter methods with a single argument.
As a consequence, stick with qualifiers if your injection target is a constructor or a
multi-argument method.

You can create your own custom qualifier annotations. Simply define an annotation and
provide the @Qualifier annotation within your definition:

Next, provide the information for the candidate bean definitions. You can add
<qualifier/> tags as sub-elements of the <bean/> tag and then specify the type and
value to match your custom qualifier annotations. The type is matched against the
fully-qualified class name of the annotation. Or, as a convenience if no risk of
conflicting names exists, you can use the short class name. Both approaches are
demonstrated in the following example.

In some cases, it may be sufficient to use an annotation without a value. This may be
useful when the annotation serves a more generic purpose and can be applied across
several different types of dependencies. For example, you may provide an offline
catalog that would be searched when no Internet connection is available. First define
the simple annotation:

<beanclass="example.SimpleMovieCatalog"><qualifier type="Offline"/><!-- inject any dependencies required by this bean --></bean>

You can also define custom qualifier annotations that accept named attributes in
addition to or instead of the simple value attribute. If multiple attribute values are
then specified on a field or parameter to be autowired, a bean definition must match
all such attribute values to be considered an autowire candidate. As an example,
consider the following annotation definition:

Finally, the bean definitions should contain matching qualifier values. This example
also demonstrates that bean meta attributes may be used instead of the
<qualifier/> sub-elements. If available, the <qualifier/> and its attributes take
precedence, but the autowiring mechanism falls back on the values provided within the
<meta/> tags if no such qualifier is present, as in the last two bean definitions in
the following example.

<?xml version="1.0" encoding="UTF-8"?><beansxmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xmlns:context="http://www.springframework.org/schema/context"xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd"><context:annotation-config/><beanclass="example.SimpleMovieCatalog"><qualifiertype="MovieQualifier"><attributekey="format"value="VHS"/><attributekey="genre"value="Action"/></qualifier><!-- inject any dependencies required by this bean --></bean><beanclass="example.SimpleMovieCatalog"><qualifiertype="MovieQualifier"><attributekey="format"value="VHS"/><attributekey="genre"value="Comedy"/></qualifier><!-- inject any dependencies required by this bean --></bean><beanclass="example.SimpleMovieCatalog"><metakey="format"value="DVD"/><metakey="genre"value="Action"/><!-- inject any dependencies required by this bean --></bean><beanclass="example.SimpleMovieCatalog"><metakey="format"value="BLURAY"/><metakey="genre"value="Comedy"/><!-- inject any dependencies required by this bean --></bean></beans>

6.9.5 Using generics as autowiring qualifiers

In addition to the @Qualifier annotation, it is also possible to use Java generic types
as an implicit form of qualification. For example, suppose you have the following
configuration:

any default-autowire-candidates pattern(s) available on the <beans/> element

the presence of @Qualifier annotations and any custom annotations registered
with the CustomAutowireConfigurer

When multiple beans qualify as autowire candidates, the determination of a "primary" is
the following: if exactly one bean definition among the candidates has a primary
attribute set to true, it will be selected.

6.9.7 @Resource

Spring also supports injection using the JSR-250 @Resource annotation on fields or
bean property setter methods. This is a common pattern in Java EE 5 and 6, for example
in JSF 1.2 managed beans or JAX-WS 2.0 endpoints. Spring supports this pattern for
Spring-managed objects as well.

@Resource takes a name attribute, and by default Spring interprets that value as the
bean name to be injected. In other words, it follows by-name semantics, as
demonstrated in this example:

If no name is specified explicitly, the default name is derived from the field name or
setter method. In case of a field, it takes the field name; in case of a setter method,
it takes the bean property name. So the following example is going to have the bean with
name "movieFinder" injected into its setter method:

The name provided with the annotation is resolved as a bean name by the
ApplicationContext of which the CommonAnnotationBeanPostProcessor is aware. The
names can be resolved through JNDI if you configure Spring’s
SimpleJndiBeanFactory
explicitly. However, it is recommended that you rely on the default behavior and simply
use Spring’s JNDI lookup capabilities to preserve the level of indirection.

In the exclusive case of @Resource usage with no explicit name specified, and similar
to @Autowired, @Resource finds a primary type match instead of a specific named bean
and resolves well-known resolvable dependencies: the BeanFactory,
ApplicationContext, ResourceLoader, ApplicationEventPublisher, and MessageSource
interfaces.

Thus in the following example, the customerPreferenceDao field first looks for a bean
named customerPreferenceDao, then falls back to a primary type match for the type
CustomerPreferenceDao. The "context" field is injected based on the known resolvable
dependency type ApplicationContext.

6.9.8 @PostConstruct and @PreDestroy

The CommonAnnotationBeanPostProcessor not only recognizes the @Resource annotation
but also the JSR-250 lifecycle annotations. Introduced in Spring 2.5, the support
for these annotations offers yet another alternative to those described in
initialization callbacks and
destruction callbacks. Provided that the
CommonAnnotationBeanPostProcessor is registered within the Spring
ApplicationContext, a method carrying one of these annotations is invoked at the same
point in the lifecycle as the corresponding Spring lifecycle interface method or
explicitly declared callback method. In the example below, the cache will be
pre-populated upon initialization and cleared upon destruction.

6.10 Classpath scanning and managed components

Most examples in this chapter use XML to specify the configuration metadata that produces
each BeanDefinition within the Spring container. The previous section
(Section 6.9, “Annotation-based container configuration”) demonstrates how to provide a lot of the configuration
metadata through source-level annotations. Even in those examples, however, the "base"
bean definitions are explicitly defined in the XML file, while the annotations only drive
the dependency injection. This section describes an option for implicitly detecting the
candidate components by scanning the classpath. Candidate components are classes that
match against a filter criteria and have a corresponding bean definition registered with
the container. This removes the need to use XML to perform bean registration; instead you
can use annotations (for example @Component), AspectJ type expressions, or your own
custom filter criteria to select which classes will have bean definitions registered with
the container.

Note

Starting with Spring 3.0, many features provided by the Spring JavaConfig project are
part of the core Spring Framework. This allows you to define beans using Java rather
than using the traditional XML files. Take a look at the @Configuration, @Bean,
@Import, and @DependsOn annotations for examples of how to use these new features.

6.10.1 @Component and further stereotype annotations

The @Repository annotation is a marker for any class that fulfills the role or
stereotype of a repository (also known as Data Access Object or DAO). Among the uses
of this marker is the automatic translation of exceptions as described in
Section 19.2.2, “Exception translation”.

Spring provides further stereotype annotations: @Component, @Service, and
@Controller. @Component is a generic stereotype for any Spring-managed component.
@Repository, @Service, and @Controller are specializations of @Component for
more specific use cases, for example, in the persistence, service, and presentation
layers, respectively. Therefore, you can annotate your component classes with
@Component, but by annotating them with @Repository, @Service, or @Controller
instead, your classes are more properly suited for processing by tools or associating
with aspects. For example, these stereotype annotations make ideal targets for
pointcuts. It is also possible that @Repository, @Service, and @Controller may
carry additional semantics in future releases of the Spring Framework. Thus, if you are
choosing between using @Component or @Service for your service layer, @Service is
clearly the better choice. Similarly, as stated above, @Repository is already
supported as a marker for automatic exception translation in your persistence layer.

6.10.2 Meta-annotations

Many of the annotations provided by Spring can be used as meta-annotations in your
own code. A meta-annotation is simply an annotation that can be applied to another
annotation. For example, the @Service annotation mentioned above is meta-annotated with
@Component:

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Component// Spring will see this and treat @Service in the same way as @Componentpublic@interface Service {
// ....
}

Meta-annotations can also be combined to create composed annotations. For example,
the @RestController annotation from Spring MVC is composed of @Controller and
@ResponseBody.

In addition, composed annotations may optionally redeclare attributes from
meta-annotations to allow user customization. This can be particularly useful when you
want to only expose a subset of the meta-annotation’s attributes. For example, the
following is a custom @Scope annotation that hardcodes the scope name to session but
still allows customization of the proxyMode.

To autodetect these classes and register the corresponding beans, you need to add
@ComponentScan to your @Configuration class, where the basePackages attribute
is a common parent package for the two classes. (Alternatively, you can specify a
comma/semicolon/space-separated list that includes the parent package of each class.)

The use of <context:component-scan> implicitly enables the functionality of
<context:annotation-config>. There is usually no need to include the
<context:annotation-config> element when using <context:component-scan>.

Note

The scanning of classpath packages requires the presence of corresponding directory
entries in the classpath. When you build JARs with Ant, make sure that you do not
activate the files-only switch of the JAR task. Also, classpath directories may not
get exposed based on security policies in some environments, e.g. standalone apps on
JDK 1.7.0_45 and higher (which requires 'Trusted-Library' setup in your manifests; see
http://stackoverflow.com/questions/19394570/java-jre-7u45-breaks-classloader-getresources).

Furthermore, the AutowiredAnnotationBeanPostProcessor and
CommonAnnotationBeanPostProcessor are both included implicitly when you use the
component-scan element. That means that the two components are autodetected and
wired together - all without any bean configuration metadata provided in XML.

Note

You can disable the registration of AutowiredAnnotationBeanPostProcessor and
CommonAnnotationBeanPostProcessor by including the annotation-config attribute
with a value of false.

6.10.4 Using filters to customize scanning

By default, classes annotated with @Component, @Repository, @Service,
@Controller, or a custom annotation that itself is annotated with @Component are the
only detected candidate components. However, you can modify and extend this behavior
simply by applying custom filters. Add them as includeFilters or excludeFilters
parameters of the @ComponentScan annotation (or as include-filter or exclude-filter
sub-elements of the component-scan element). Each filter element requires the type
and expression attributes. The following table describes the filtering options.

Table 6.5. Filter Types

Filter Type

Example Expression

Description

annotation (default)

org.example.SomeAnnotation

An annotation to be present at the type level in target components.

assignable

org.example.SomeClass

A class (or interface) that the target components are assignable to (extend/implement).

aspectj

org.example..*Service+

An AspectJ type expression to be matched by the target components.

regex

org\.example\.Default.*

A regex expression to be matched by the target components class names.

custom

org.example.MyTypeFilter

A custom implementation of the org.springframework.core.type .TypeFilter interface.

The following example shows the configuration ignoring all @Repository annotations
and using "stub" repositories instead.

You can also disable the default filters by setting useDefaultFilters=false on the annotation or
providing use-default-filters="false" as an attribute of the <component-scan/> element. This
will in effect disable automatic detection of classes annotated with @Component, @Repository,
@Service, @Controller, or @Configuration.

6.10.5 Defining bean metadata within components

Spring components can also contribute bean definition metadata to the container. You do
this with the same @Bean annotation used to define bean metadata within @Configuration
annotated classes. Here is a simple example:

This class is a Spring component that has application-specific code contained in its
doWork() method. However, it also contributes a bean definition that has a factory
method referring to the method publicInstance(). The @Bean annotation identifies the
factory method and other bean definition properties, such as a qualifier value through
the @Qualifier annotation. Other method level annotations that can be specified are
@Scope, @Lazy, and custom qualifier annotations.

Tip

In addition to its role for component initialization, the @Lazy annotation may also be
placed on injection points marked with @Autowired or @Inject. In this context, it
leads to the injection of a lazy-resolution proxy.

Autowired fields and methods are supported as previously discussed, with additional
support for autowiring of @Bean methods:

The example autowires the String method parameter country to the value of the Age
property on another bean named privateInstance. A Spring Expression Language element
defines the value of the property through the notation #{ <expression> }. For @Value
annotations, an expression resolver is preconfigured to look for bean names when
resolving expression text.

The @Bean methods in a Spring component are processed differently than their
counterparts inside a Spring @Configuration class. The difference is that @Component
classes are not enhanced with CGLIB to intercept the invocation of methods and fields.
CGLIB proxying is the means by which invoking methods or fields within @Bean methods
in @Configuration classes creates bean metadata references to collaborating objects;
such methods are not invoked with normal Java semantics but rather go through the
container in order to provide the usual lifecycle management and proxying of Spring
beans even when referring to other beans via programmatic calls to @Bean methods.
In contrast, invoking a method or field in an @Bean method within a plain @Component
class has standard Java semantics, with no special CGLIB processing or other
constraints applying.

Note

You may declare @Bean methods as static, allowing for them to be called without
creating their containing configuration class as an instance. This makes particular
sense when defining post-processor beans, e.g. of type BeanFactoryPostProcessor or
BeanPostProcessor, since such beans will get initialized early in the container
lifecycle and should avoid triggering other parts of the configuration at that point.

Note that calls to static @Bean methods will never get intercepted by the container,
not even within @Configuration classes (see above). This is due to technical
limitations: CGLIB subclassing can only override non-static methods. As a consequence,
a direct call to another @Bean method will have standard Java semantics, resulting
in an independent instance being returned straight from the factory method itself.

The Java language visibility of @Bean methods does not have an immediate impact on
the resulting bean definition in Spring’s container. You may freely declare your
factory methods as you see fit in non-@Configuration classes and also for static
methods anywhere. However, regular @Bean methods in @Configuration classes need
to be overridable, i.e. they must not be declared as private or final.

@Bean methods will also be discovered on base classes of a given component or
configuration class, as well as on Java 8 default methods declared in interfaces
implemented by the component or configuration class. This allows for a lot of
flexibility in composing complex configuration arrangements, with even multiple
inheritance being possible through Java 8 default methods as of Spring 4.2.

Finally, note that a single class may hold multiple @Bean methods for the same
bean, as an arrangement of multiple factory methods to use depending on available
dependencies at runtime. This is the same algorithm as for choosing the "greediest"
constructor or factory method in other configuration scenarios: The variant with
the largest number of satisfiable dependencies will be picked at construction time,
analogous to how the container selects between multiple @Autowired constructors.

6.10.6 Naming autodetected components

When a component is autodetected as part of the scanning process, its bean name is
generated by the BeanNameGenerator strategy known to that scanner. By default, any
Spring stereotype annotation (@Component, @Repository, @Service, and
@Controller) that contains a namevalue will thereby provide that name to the
corresponding bean definition.

If such an annotation contains no namevalue or for any other detected component (such
as those discovered by custom filters), the default bean name generator returns the
uncapitalized non-qualified class name. For example, if the following two components
were detected, the names would be myMovieLister and movieFinderImpl:

If you do not want to rely on the default bean-naming strategy, you can provide a custom
bean-naming strategy. First, implement the
BeanNameGenerator
interface, and be sure to include a default no-arg constructor. Then, provide the
fully-qualified class name when configuring the scanner:

As a general rule, consider specifying the name with the annotation whenever other
components may be making explicit references to it. On the other hand, the
auto-generated names are adequate whenever the container is responsible for wiring.

6.10.7 Providing a scope for autodetected components

As with Spring-managed components in general, the default and most common scope for
autodetected components is singleton. However, sometimes you need a different scope
which can be specified via the @Scope annotation. Simply provide the name of the scope
within the annotation:

To provide a custom strategy for scope resolution rather than relying on the
annotation-based approach, implement the
ScopeMetadataResolver
interface, and be sure to include a default no-arg constructor. Then, provide the
fully-qualified class name when configuring the scanner:

When using certain non-singleton scopes, it may be necessary to generate proxies for the
scoped objects. The reasoning is described in the section called “Scoped beans as dependencies”.
For this purpose, a scoped-proxy attribute is available on the component-scan
element. The three possible values are: no, interfaces, and targetClass. For example,
the following configuration will result in standard JDK dynamic proxies:

6.10.8 Providing qualifier metadata with annotations

The @Qualifier annotation is discussed in Section 6.9.4, “Fine-tuning annotation-based autowiring with qualifiers”.
The examples in that section demonstrate the use of the @Qualifier annotation and
custom qualifier annotations to provide fine-grained control when you resolve autowire
candidates. Because those examples were based on XML bean definitions, the qualifier
metadata was provided on the candidate bean definitions using the qualifier or meta
sub-elements of the bean element in the XML. When relying upon classpath scanning for
autodetection of components, you provide the qualifier metadata with type-level
annotations on the candidate class. The following three examples demonstrate this
technique:

As with most annotation-based alternatives, keep in mind that the annotation metadata is
bound to the class definition itself, while the use of XML allows for multiple beans
of the same type to provide variations in their qualifier metadata, because that
metadata is provided per-instance rather than per-class.

6.11 Using JSR 330 Standard Annotations

Starting with Spring 3.0, Spring offers support for JSR-330 standard annotations
(Dependency Injection). Those annotations are scanned in the same way as the Spring
annotations. You just need to have the relevant jars in your classpath.

As with @Autowired, it is possible to use @Inject at the field level, method level
and constructor-argument level. Furthermore, you may declare your injection point as a
Provider, allowing for on-demand access to beans of shorter scopes or lazy access to
other beans through a Provider.get() call. As a variant of the example above:

In contrast to @Component, the JSR-330 @Named annotation is not composable.
Please use Spring’s stereotype model for building custom component annotations.

6.11.3 Limitations of JSR-330 standard annotations

When working with standard annotations, it is important to know that some significant
features are not available as shown in the table below:

Table 6.6. Spring component model elements vs. JSR-330 variants

Spring

javax.inject.*

javax.inject restrictions / comments

@Autowired

@Inject

@Inject has no 'required' attribute; can be used with Java 8’s Optional instead.

@Component

@Named

JSR-330 does not provide a composable model, just a way to identify named components.

@Scope("singleton")

@Singleton

The JSR-330 default scope is like Spring’s prototype. However, in order to keep it
consistent with Spring’s general defaults, a JSR-330 bean declared in the Spring
container is a singleton by default. In order to use a scope other than singleton,
you should use Spring’s @Scope annotation. javax.inject also provides a
@Scope annotation.
Nevertheless, this one is only intended to be used for creating your own annotations.

@Qualifier

@Qualifier / @Named

javax.inject.Qualifier is just a meta-annotation for building custom qualifiers.
Concrete String qualifiers (like Spring’s @Qualifier with a value) can be associated
through javax.inject.Named.

@Value

-

no equivalent

@Required

-

no equivalent

@Lazy

-

no equivalent

ObjectFactory

Provider

javax.inject.Provider is a direct alternative to Spring’s ObjectFactory,
just with a shorter get() method name. It can also be used in combination with
Spring’s @Autowired or with non-annotated constructors and setter methods.

6.12 Java-based container configuration

6.12.1 Basic concepts: @Bean and @Configuration

The central artifacts in Spring’s new Java-configuration support are
@Configuration-annotated classes and @Bean-annotated methods.

The @Bean annotation is used to indicate that a method instantiates, configures and
initializes a new object to be managed by the Spring IoC container. For those familiar
with Spring’s <beans/> XML configuration the @Bean annotation plays the same role as
the <bean/> element. You can use @Bean annotated methods with any Spring
@Component, however, they are most often used with @Configuration beans.

Annotating a class with @Configuration indicates that its primary purpose is as a
source of bean definitions. Furthermore, @Configuration classes allow inter-bean
dependencies to be defined by simply calling other @Bean methods in the same class.
The simplest possible @Configuration class would read as follows:

When @Bean methods are declared within classes that are not annotated with
@Configuration they are referred to as being processed in a 'lite' mode. For example,
bean methods declared in a @Component or even in a plain old class will be
considered 'lite'.

Unlike full @Configuration, lite @Bean methods cannot easily declare inter-bean
dependencies. Usually one @Bean method should not invoke another @Bean method when
operating in 'lite' mode.

Only using @Bean methods within @Configuration classes is a recommended approach of
ensuring that 'full' mode is always used. This will prevent the same @Bean method from
accidentally being invoked multiple times and helps to reduce subtle bugs that can be
hard to track down when operating in 'lite' mode.

The @Bean and @Configuration annotations will be discussed in depth in the sections
below. First, however, we’ll cover the various ways of creating a spring container using
Java-based configuration.

The sections below document Spring’s AnnotationConfigApplicationContext, new in Spring
3.0. This versatile ApplicationContext implementation is capable of accepting not only
@Configuration classes as input, but also plain @Component classes and classes
annotated with JSR-330 metadata.

When @Configuration classes are provided as input, the @Configuration class itself
is registered as a bean definition, and all declared @Bean methods within the class
are also registered as bean definitions.

When @Component and JSR-330 classes are provided, they are registered as bean
definitions, and it is assumed that DI metadata such as @Autowired or @Inject are
used within those classes where necessary.

Simple construction

In much the same way that Spring XML files are used as input when instantiating a
ClassPathXmlApplicationContext, @Configuration classes may be used as input when
instantiating an AnnotationConfigApplicationContext. This allows for completely
XML-free usage of the Spring container:

As mentioned above, AnnotationConfigApplicationContext is not limited to working only
with @Configuration classes. Any @Component or JSR-330 annotated class may be supplied
as input to the constructor. For example:

The above assumes that MyServiceImpl, Dependency1 and Dependency2 use Spring
dependency injection annotations such as @Autowired.

Building the container programmatically using register(Class<?>…​)

An AnnotationConfigApplicationContext may be instantiated using a no-arg constructor
and then configured using the register() method. This approach is particularly useful
when programmatically building an AnnotationConfigApplicationContext.

Enabling component scanning with scan(String…​)

Experienced Spring users will be familiar with the XML declaration equivalent from
Spring’s context: namespace

<beans><context:component-scanbase-package="com.acme"/></beans>

In the example above, the com.acme package will be scanned, looking for any
@Component-annotated classes, and those classes will be registered as Spring bean
definitions within the container. AnnotationConfigApplicationContext exposes the
scan(String…​) method to allow for the same component-scanning functionality:

Remember that @Configuration classes are meta-annotated
with @Component, so they are candidates for component-scanning! In the example above,
assuming that AppConfig is declared within the com.acme package (or any package
underneath), it will be picked up during the call to scan(), and upon refresh() all
its @Bean methods will be processed and registered as bean definitions within the
container.

Support for web applications with AnnotationConfigWebApplicationContext

A WebApplicationContext variant of AnnotationConfigApplicationContext is available
with AnnotationConfigWebApplicationContext. This implementation may be used when
configuring the Spring ContextLoaderListener servlet listener, Spring MVC
DispatcherServlet, etc. What follows is a web.xml snippet that configures a typical
Spring MVC web application. Note the use of the contextClass context-param and
init-param:

<web-app><!-- Configure ContextLoaderListener to use AnnotationConfigWebApplicationContext
instead of the default XmlWebApplicationContext --><context-param><param-name>contextClass</param-name><param-value>
org.springframework.web.context.support.AnnotationConfigWebApplicationContext
</param-value></context-param><!-- Configuration locations must consist of one or more comma- or space-delimited
fully-qualified @Configuration classes. Fully-qualified packages may also be
specified for component-scanning --><context-param><param-name>contextConfigLocation</param-name><param-value>com.acme.AppConfig</param-value></context-param><!-- Bootstrap the root application context as usual using ContextLoaderListener --><listener><listener-class>org.springframework.web.context.ContextLoaderListener</listener-class></listener><!-- Declare a Spring MVC DispatcherServlet as usual --><servlet><servlet-name>dispatcher</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class><!-- Configure DispatcherServlet to use AnnotationConfigWebApplicationContext
instead of the default XmlWebApplicationContext --><init-param><param-name>contextClass</param-name><param-value>
org.springframework.web.context.support.AnnotationConfigWebApplicationContext
</param-value></init-param><!-- Again, config locations must consist of one or more comma- or space-delimited
and fully-qualified @Configuration classes --><init-param><param-name>contextConfigLocation</param-name><param-value>com.acme.web.MvcConfig</param-value></init-param></servlet><!-- map all requests for /app/* to the dispatcher servlet --><servlet-mapping><servlet-name>dispatcher</servlet-name><url-pattern>/app/*</url-pattern></servlet-mapping></web-app>

6.12.3 Using the @Bean annotation

@Bean is a method-level annotation and a direct analog of the XML <bean/> element.
The annotation supports some of the attributes offered by <bean/>, such as:
init-method,
destroy-method,
autowiring and name.

You can use the @Bean annotation in a @Configuration-annotated or in a
@Component-annotated class.

Declaring a bean

To declare a bean, simply annotate a method with the @Bean annotation. You use this
method to register a bean definition within an ApplicationContext of the type
specified as the method’s return value. By default, the bean name will be the same as
the method name. The following is a simple example of a @Bean method declaration:

Both declarations make a bean named transferService available in the
ApplicationContext, bound to an object instance of type TransferServiceImpl:

transferService -> com.acme.TransferServiceImpl

Bean dependencies

A @Bean annotated method can have an arbitrary number of parameters describing the
dependencies required to build that bean. For instance if our TransferService
requires an AccountRepository we can materialize that dependency via a method
parameter:

The resolution mechanism is pretty much identical to constructor-based dependency
injection, see the relevant section for more details.

Receiving lifecycle callbacks

Any classes defined with the @Bean annotation support the regular lifecycle callbacks
and can use the @PostConstruct and @PreDestroy annotations from JSR-250, see
JSR-250 annotations for further
details.

The regular Spring lifecycle callbacks are fully supported as
well. If a bean implements InitializingBean, DisposableBean, or Lifecycle, their
respective methods are called by the container.

By default, beans defined using Java config that have a public close or shutdown
method are automatically enlisted with a destruction callback. If you have a public
close or shutdown method and you do not wish for it to be called when the container
shuts down, simply add @Bean(destroyMethod="") to your bean definition to disable the
default (inferred) mode.

You may want to do that by default for a resource that you acquire via JNDI as its
lifecycle is managed outside the application. In particular, make sure to always do it
for a DataSource as it is known to be problematic on Java EE application servers.

Also, with @Bean methods, you will typically choose to use programmatic JNDI lookups:
either using Spring’s JndiTemplate/JndiLocatorDelegate helpers or straight JNDI
InitialContext usage, but not the JndiObjectFactoryBean variant which would force
you to declare the return type as the FactoryBean type instead of the actual target
type, making it harder to use for cross-reference calls in other @Bean methods that
intend to refer to the provided resource here.

Of course, in the case of Foo above, it would be equally as valid to call the init()
method directly during construction:

@Scope and scoped-proxy

Spring offers a convenient way of working with scoped dependencies through
scoped proxies. The easiest way to create such
a proxy when using the XML configuration is the <aop:scoped-proxy/> element.
Configuring your beans in Java with a @Scope annotation offers equivalent support with
the proxyMode attribute. The default is no proxy ( ScopedProxyMode.NO), but you can
specify ScopedProxyMode.TARGET_CLASS or ScopedProxyMode.INTERFACES.

If you port the scoped proxy example from the XML reference documentation (see preceding
link) to our @Bean using Java, it would look like the following:

Bean aliasing

As discussed in Section 6.3.1, “Naming beans”, it is sometimes desirable to give a single bean
multiple names, otherwise known asbean aliasing. The name attribute of the @Bean
annotation accepts a String array for this purpose.

6.12.4 Using the @Configuration annotation

@Configuration is a class-level annotation indicating that an object is a source of
bean definitions. @Configuration classes declare beans via public @Bean annotated
methods. Calls to @Bean methods on @Configuration classes can also be used to define
inter-bean dependencies. See Section 6.12.1, “Basic concepts: @Bean and @Configuration” for a general introduction.

Injecting inter-bean dependencies

When @Beans have dependencies on one another, expressing that dependency is as simple
as having one bean method call another:

In the example above, the foo bean receives a reference to bar via constructor
injection.

Note

This method of declaring inter-bean dependencies only works when the @Bean method is
declared within a @Configuration class. You cannot declare inter-bean dependencies
using plain @Component classes.

Lookup method injection

As noted earlier, lookup method injection is an
advanced feature that you should use rarely. It is useful in cases where a
singleton-scoped bean has a dependency on a prototype-scoped bean. Using Java for this
type of configuration provides a natural means for implementing this pattern.

publicabstractclass CommandManager {
public Object process(Object commandState) {
// grab a new instance of the appropriate Command interface
Command command = createCommand();
// set the state on the (hopefully brand new) Command instance
command.setState(commandState);
return command.execute();
}
// okay... but where is the implementation of this method?protectedabstract Command createCommand();
}

Using Java-configuration support , you can create a subclass of CommandManager where
the abstract createCommand() method is overridden in such a way that it looks up a new
(prototype) command object:

clientDao() has been called once in clientService1() and once in clientService2().
Since this method creates a new instance of ClientDaoImpl and returns it, you would
normally expect having 2 instances (one for each service). That definitely would be
problematic: in Spring, instantiated beans have a singleton scope by default. This is
where the magic comes in: All @Configuration classes are subclassed at startup-time
with CGLIB. In the subclass, the child method checks the container first for any
cached (scoped) beans before it calls the parent method and creates a new instance. Note
that as of Spring 3.2, it is no longer necessary to add CGLIB to your classpath because
CGLIB classes have been repackaged under org.springframework and included directly
within the spring-core JAR.

Note

The behavior could be different according to the scope of your bean. We are talking
about singletons here.

Tip

There are a few restrictions due to the fact that CGLIB dynamically adds features at
startup-time, in particular that configuration classes must not be final and need to
have a default constructor with no arguments.

If you prefer to avoid any CGLIB-imposed limitations, consider declaring your @Bean
methods on non-@Configuration classes, e.g. on plain @Component classes instead.
Cross-method calls between @Bean methods won’t get intercepted then, so you’ll have
to exclusively rely on dependency injection at the field or method level there.

6.12.5 Composing Java-based configurations

Using the @Import annotation

Much as the <import/> element is used within Spring XML files to aid in modularizing
configurations, the @Import annotation allows for loading @Bean definitions from
another configuration class:

Now, rather than needing to specify both ConfigA.class and ConfigB.class when
instantiating the context, only ConfigB needs to be supplied explicitly:

publicstaticvoid main(String[] args) {
ApplicationContext ctx = new AnnotationConfigApplicationContext(ConfigB.class);
// now both beans A and B will be available...
A a = ctx.getBean(A.class);
B b = ctx.getBean(B.class);
}

This approach simplifies container instantiation, as only one class needs to be dealt
with, rather than requiring the developer to remember a potentially large number of
@Configuration classes during construction.

Tip

As of Spring Framework 4.2, @Import also supports references to regular component
classes, analogous to the AnnotationConfigApplicationContext.register method.
This is particularly useful if you’d like to avoid component scanning, using a few
configuration classes as entry points for explicitly defining all your components.

Injecting dependencies on imported @Bean definitions

The example above works, but is simplistic. In most practical scenarios, beans will have
dependencies on one another across configuration classes. When using XML, this is not an
issue, per se, because there is no compiler involved, and one can simply declare
ref="someBean" and trust that Spring will work it out during container initialization.
Of course, when using @Configuration classes, the Java compiler places constraints on
the configuration model, in that references to other beans must be valid Java syntax.

Fortunately, solving this problem is simple. As we already discussed,
@Bean method can have an arbitrary number of parameters describing the bean
dependencies. Let’s consider a more real-world scenario with several @Configuration
classes, each depending on beans declared in the others:

There is another way to achieve the same result. Remember that @Configuration classes are
ultimately just another bean in the container: This means that they can take advantage of
@Autowired and @Value injection etc just like any other bean!

Warning

Make sure that the dependencies you inject that way are of the simplest kind only. @Configuration
classes are processed quite early during the initialization of the context and forcing a dependency
to be injected this way may lead to unexpected early initialization. Whenever possible, resort to
parameter-based injection as in the example above.

Also, be particularly careful with BeanPostProcessor and BeanFactoryPostProcessor definitions
via @Bean. Those should usually be declared as static @Bean methods, not triggering the
instantiation of their containing configuration class. Otherwise, @Autowired and @Value won’t
work on the configuration class itself since it is being created as a bean instance too early.

In the scenario above, using @Autowired works well and provides the desired
modularity, but determining exactly where the autowired bean definitions are declared is
still somewhat ambiguous. For example, as a developer looking at ServiceConfig, how do
you know exactly where the @Autowired AccountRepository bean is declared? It’s not
explicit in the code, and this may be just fine. Remember that the
Spring Tool Suite provides tooling that
can render graphs showing how everything is wired up - that may be all you need. Also,
your Java IDE can easily find all declarations and uses of the AccountRepository type,
and will quickly show you the location of @Bean methods that return that type.

In cases where this ambiguity is not acceptable and you wish to have direct navigation
from within your IDE from one @Configuration class to another, consider autowiring the
configuration classes themselves:

In the situation above, it is completely explicit where AccountRepository is defined.
However, ServiceConfig is now tightly coupled to RepositoryConfig; that’s the
tradeoff. This tight coupling can be somewhat mitigated by using interface-based or
abstract class-based @Configuration classes. Consider the following:

Now ServiceConfig is loosely coupled with respect to the concrete
DefaultRepositoryConfig, and built-in IDE tooling is still useful: it will be easy for
the developer to get a type hierarchy of RepositoryConfig implementations. In this
way, navigating @Configuration classes and their dependencies becomes no different
than the usual process of navigating interface-based code.

Conditionally include @Configuration classes or @Bean methods

It is often useful to conditionally enable or disable a complete @Configuration class,
or even individual @Bean methods, based on some arbitrary system state. One common
example of this is to use the @Profile annotation to activate beans only when a specific
profile has been enabled in the Spring Environment (see Section 6.13.1, “Bean definition profiles”
for details).

The @Profile annotation is actually implemented using a much more flexible annotation
called @Conditional.
The @Conditional annotation indicates specific
org.springframework.context.annotation.Condition implementations that should be
consulted before a @Bean is registered.

Implementations of the Condition interface simply provide a matches(…​)
method that returns true or false. For example, here is the actual
Condition implementation used for @Profile:

Combining Java and XML configuration

Spring’s @Configuration class support does not aim to be a 100% complete replacement
for Spring XML. Some facilities such as Spring XML namespaces remain an ideal way to
configure the container. In cases where XML is convenient or necessary, you have a
choice: either instantiate the container in an "XML-centric" way using, for example,
ClassPathXmlApplicationContext, or in a "Java-centric" fashion using
AnnotationConfigApplicationContext and the @ImportResource annotation to import XML
as needed.

XML-centric use of @Configuration classes

It may be preferable to bootstrap the Spring container from XML and include
@Configuration classes in an ad-hoc fashion. For example, in a large existing codebase
that uses Spring XML, it will be easier to create @Configuration classes on an
as-needed basis and include them from the existing XML files. Below you’ll find the
options for using @Configuration classes in this kind of "XML-centric" situation.

Remember that @Configuration classes are ultimately just bean definitions in the
container. In this example, we create a @Configuration class named AppConfig and
include it within system-test-config.xml as a <bean/> definition. Because
<context:annotation-config/> is switched on, the container will recognize the
@Configuration annotation and process the @Bean methods declared in AppConfig
properly.

<beans><!-- enable processing of annotations such as @Autowired and @Configuration --><context:annotation-config/><context:property-placeholderlocation="classpath:/com/acme/jdbc.properties"/><beanclass="com.acme.AppConfig"/><beanclass="org.springframework.jdbc.datasource.DriverManagerDataSource"><propertyname="url"value="${jdbc.url}"/><propertyname="username"value="${jdbc.username}"/><propertyname="password"value="${jdbc.password}"/></bean></beans>

In system-test-config.xml above, the AppConfig<bean/> does not declare an id
element. While it would be acceptable to do so, it is unnecessary given that no other
bean will ever refer to it, and it is unlikely that it will be explicitly fetched from
the container by name. Likewise with the DataSource bean - it is only ever autowired
by type, so an explicit bean id is not strictly required.

Because @Configuration is meta-annotated with @Component, @Configuration-annotated
classes are automatically candidates for component scanning. Using the same scenario as
above, we can redefine system-test-config.xml to take advantage of component-scanning.
Note that in this case, we don’t need to explicitly declare
<context:annotation-config/>, because <context:component-scan/> enables the same
functionality.

system-test-config.xml:

<beans><!-- picks up and registers AppConfig as a bean definition --><context:component-scanbase-package="com.acme"/><context:property-placeholderlocation="classpath:/com/acme/jdbc.properties"/><beanclass="org.springframework.jdbc.datasource.DriverManagerDataSource"><propertyname="url"value="${jdbc.url}"/><propertyname="username"value="${jdbc.username}"/><propertyname="password"value="${jdbc.password}"/></bean></beans>

@Configuration class-centric use of XML with @ImportResource

In applications where @Configuration classes are the primary mechanism for configuring
the container, it will still likely be necessary to use at least some XML. In these
scenarios, simply use @ImportResource and define only as much XML as is needed. Doing
so achieves a "Java-centric" approach to configuring the container and keeps XML to a
bare minimum.

6.13 Environment abstraction

The Environment
is an abstraction integrated in the container that models two key
aspects of the application environment: profiles
and properties.

A profile is a named, logical group of bean definitions to be registered with the
container only if the given profile is active. Beans may be assigned to a profile
whether defined in XML or via annotations. The role of the Environment object with
relation to profiles is in determining which profiles (if any) are currently active,
and which profiles (if any) should be active by default.

Properties play an important role in almost all applications, and may originate from
a variety of sources: properties files, JVM system properties, system environment
variables, JNDI, servlet context parameters, ad-hoc Properties objects, Maps, and so
on. The role of the Environment object with relation to properties is to provide the
user with a convenient service interface for configuring property sources and resolving
properties from them.

6.13.1 Bean definition profiles

Bean definition profiles is a mechanism in the core container that allows for
registration of different beans in different environments. The word environment
can mean different things to different users and this feature can help with many
use cases, including:

working against an in-memory datasource in development vs looking up that same
datasource from JNDI when in QA or production

registering monitoring infrastructure only when deploying an application into a
performance environment

Let’s now consider how this application will be deployed into a QA or production
environment, assuming that the datasource for the application will be registered
with the production application server’s JNDI directory. Our dataSource bean
now looks like this:

The problem is how to switch between using these two variations based on the
current environment. Over time, Spring users have devised a number of ways to
get this done, usually relying on a combination of system environment variables
and XML <import/> statements containing ${placeholder} tokens that resolve
to the correct configuration file path depending on the value of an environment
variable. Bean definition profiles is a core container feature that provides a
solution to this problem.

If we generalize the example use case above of environment-specific bean
definitions, we end up with the need to register certain bean definitions in
certain contexts, while not in others. You could say that you want to register a
certain profile of bean definitions in situation A, and a different profile in
situation B. Let’s first see how we can update our configuration to reflect
this need.

@Profile

The @Profile
annotation allows you to indicate that a component is eligible for registration
when one or more specified profiles are active. Using our example above, we
can rewrite the dataSource configuration as follows:

As mentioned before, with @Bean methods, you will typically choose to use programmatic
JNDI lookups: either using Spring’s JndiTemplate/JndiLocatorDelegate helpers or the
straight JNDI InitialContext usage shown above, but not the JndiObjectFactoryBean
variant which would force you to declare the return type as the FactoryBean type.

@Profile can be used as a meta-annotation for the purpose
of creating a custom composed annotation. The following example defines a custom
@Production annotation that can be used as a drop-in replacement for
@Profile("production"):

If a @Configuration class is marked with @Profile, all of the @Bean methods and
@Import annotations associated with that class will be bypassed unless one or more of
the specified profiles are active. If a @Component or @Configuration class is marked
with @Profile({"p1", "p2"}), that class will not be registered/processed unless
profiles 'p1' and/or 'p2' have been activated. If a given profile is prefixed with the
NOT operator (!), the annotated element will be registered if the profile is not
active. For example, given @Profile({"p1", "!p2"}), registration will occur if profile
'p1' is active or if profile 'p2' is not active.

6.13.2 XML bean definition profiles

The XML counterpart is the profile attribute of the <beans> element. Our sample
configuration above can be rewritten in two XML files as follows:

The spring-bean.xsd has been constrained to allow such elements only as the
last ones in the file. This should help provide flexibility without incurring
clutter in the XML files.

Activating a profile

Now that we have updated our configuration, we still need to instruct Spring which
profile is active. If we started our sample application right now, we would see
a NoSuchBeanDefinitionException thrown, because the container could not find
the Spring bean named dataSource.

Activating a profile can be done in several ways, but the most straightforward is to do
it programmatically against the Environment API which is available via an
ApplicationContext:

Note that profiles are not an "either-or" proposition; it is possible to activate multiple
profiles at once. Programmatically, simply provide multiple profile names to the
setActiveProfiles() method, which accepts String…​ varargs:

ctx.getEnvironment().setActiveProfiles("profile1", "profile2");

Declaratively, spring.profiles.active may accept a comma-separated list of profile names:

-Dspring.profiles.active="profile1,profile2"

Default profile

The default profile represents the profile that is enabled by default. Consider the
following:

If no profile is active, the dataSource above will be created; this can be
seen as a way to provide a default definition for one or more beans. If any
profile is enabled, the default profile will not apply.

The name of the default profile can be changed using setDefaultProfiles() on
the Environment or declaratively using the spring.profiles.default property.

In the snippet above, we see a high-level way of asking Spring whether the foo property is
defined for the current environment. To answer this question, the Environment object performs
a search over a set of PropertySource
objects. A PropertySource is a simple abstraction over any source of key-value pairs, and
Spring’s StandardEnvironment
is configured with two PropertySource objects — one representing the set of JVM system properties
(a laSystem.getProperties()) and one representing the set of system environment variables
(a laSystem.getenv()).

Note

These default property sources are present for StandardEnvironment, for use in standalone
applications. StandardServletEnvironment
is populated with additional default property sources including servlet config and servlet
context parameters. StandardPortletEnvironment
similarly has access to portlet config and portlet context parameters as property sources.
Both can optionally enable a JndiPropertySource.
See the javadocs for details.

Concretely, when using the StandardEnvironment, the call to env.containsProperty("foo")
will return true if a foo system property or foo environment variable is present at
runtime.

Tip

The search performed is hierarchical. By default, system properties have precedence over
environment variables, so if the foo property happens to be set in both places during
a call to env.getProperty("foo"), the system property value will 'win' and be returned
preferentially over the environment variable. Note that property values will not get merged
but rather completely overridden by a preceding entry.

Most importantly, the entire mechanism is configurable. Perhaps you have a custom source
of properties that you’d like to integrate into this search. No problem — simply implement
and instantiate your own PropertySource and add it to the set of PropertySources for the
current Environment:

In the code above, MyPropertySource has been added with highest precedence in the
search. If it contains a foo property, it will be detected and returned ahead of
any foo property in any other PropertySource. The
MutablePropertySources
API exposes a number of methods that allow for precise manipulation of the set of
property sources.

6.13.4 @PropertySource

The @PropertySource
annotation provides a convenient and declarative mechanism for adding a PropertySource
to Spring’s Environment.

Given a file "app.properties" containing the key/value pair testbean.name=myTestBean,
the following @Configuration class uses @PropertySource in such a way that
a call to testBean.getName() will return "myTestBean".

Assuming that "my.placeholder" is present in one of the property sources already
registered, e.g. system properties or environment variables, the placeholder will
be resolved to the corresponding value. If not, then "default/path" will be used
as a default. If no default is specified and a property cannot be resolved, an
IllegalArgumentException will be thrown.

6.13.5 Placeholder resolution in statements

Historically, the value of placeholders in elements could be resolved only against
JVM system properties or environment variables. No longer is this the case. Because
the Environment abstraction is integrated throughout the container, it’s easy to
route resolution of placeholders through it. This means that you may configure the
resolution process in any way you like: change the precedence of searching through
system properties and environment variables, or remove them entirely; add your
own property sources to the mix as appropriate.

Concretely, the following statement works regardless of where the customer
property is defined, as long as it is available in the Environment:

6.14 Registering a LoadTimeWeaver

The LoadTimeWeaver is used by Spring to dynamically transform classes as they are
loaded into the Java virtual machine (JVM).

To enable load-time weaving add the @EnableLoadTimeWeaving to one of your
@Configuration classes:

@Configuration@EnableLoadTimeWeavingpublicclass AppConfig {
}

Alternatively for XML configuration use the context:load-time-weaver element:

<beans><context:load-time-weaver/></beans>

Once configured for the ApplicationContext. Any bean within that ApplicationContext
may implement LoadTimeWeaverAware, thereby receiving a reference to the load-time
weaver instance. This is particularly useful in combination with Spring’s JPA
support where load-time weaving may be necessary for JPA class transformation. Consult
the LocalContainerEntityManagerFactoryBean javadocs for more detail. For more on
AspectJ load-time weaving, see Section 10.8.4, “Load-time weaving with AspectJ in the Spring Framework”.

6.15 Additional Capabilities of the ApplicationContext

As was discussed in the chapter introduction, the org.springframework.beans.factory
package provides basic functionality for managing and manipulating beans, including in a
programmatic way. The org.springframework.context package adds the
ApplicationContext
interface, which extends the BeanFactory interface, in addition to extending other
interfaces to provide additional functionality in a more application
framework-oriented style. Many people use the ApplicationContext in a completely
declarative fashion, not even creating it programmatically, but instead relying on
support classes such as ContextLoader to automatically instantiate an
ApplicationContext as part of the normal startup process of a Java EE web application.

To enhance BeanFactory functionality in a more framework-oriented style the context
package also provides the following functionality:

Access to messages in i18n-style, through the MessageSource interface.

Access to resources, such as URLs and files, through the ResourceLoader interface.

Event publication to namely beans implementing the ApplicationListener interface,
through the use of the ApplicationEventPublisher interface.

Loading of multiple (hierarchical) contexts, allowing each to be focused on one
particular layer, such as the web layer of an application, through the
HierarchicalBeanFactory interface.

6.15.1 Internationalization using MessageSource

The ApplicationContext interface extends an interface called MessageSource, and
therefore provides internationalization (i18n) functionality. Spring also provides the
interface HierarchicalMessageSource, which can resolve messages hierarchically.
Together these interfaces provide the foundation upon which Spring effects message
resolution. The methods defined on these interfaces include:

String getMessage(String code, Object[] args, String default, Locale loc): The basic
method used to retrieve a message from the MessageSource. When no message is found
for the specified locale, the default message is used. Any arguments passed in become
replacement values, using the MessageFormat functionality provided by the standard
library.

String getMessage(String code, Object[] args, Locale loc): Essentially the same as
the previous method, but with one difference: no default message can be specified; if
the message cannot be found, a NoSuchMessageException is thrown.

String getMessage(MessageSourceResolvable resolvable, Locale locale): All properties
used in the preceding methods are also wrapped in a class named
MessageSourceResolvable, which you can use with this method.

When an ApplicationContext is loaded, it automatically searches for a MessageSource
bean defined in the context. The bean must have the name messageSource. If such a bean
is found, all calls to the preceding methods are delegated to the message source. If no
message source is found, the ApplicationContext attempts to find a parent containing a
bean with the same name. If it does, it uses that bean as the MessageSource. If the
ApplicationContext cannot find any source for messages, an empty
DelegatingMessageSource is instantiated in order to be able to accept calls to the
methods defined above.

Spring provides two MessageSource implementations, ResourceBundleMessageSource and
StaticMessageSource. Both implement HierarchicalMessageSource in order to do nested
messaging. The StaticMessageSource is rarely used but provides programmatic ways to
add messages to the source. The ResourceBundleMessageSource is shown in the following
example:

In the example it is assumed you have three resource bundles defined in your classpath
called format, exceptions and windows. Any request to resolve a message will be
handled in the JDK standard way of resolving messages through ResourceBundles. For the
purposes of the example, assume the contents of two of the above resource bundle files
are…​

A program to execute the MessageSource functionality is shown in the next example.
Remember that all ApplicationContext implementations are also MessageSource
implementations and so can be cast to the MessageSource interface.

So to summarize, the MessageSource is defined in a file called beans.xml, which
exists at the root of your classpath. The messageSource bean definition refers to a
number of resource bundles through its basenames property. The three files that are
passed in the list to the basenames property exist as files at the root of your
classpath and are called format.properties, exceptions.properties, and
windows.properties respectively.

The next example shows arguments passed to the message lookup; these arguments will be
converted into Strings and inserted into placeholders in the lookup message.

<beans><!-- this MessageSource is being used in a web application --><beanid="messageSource"class="org.springframework.context.support.ResourceBundleMessageSource"><propertyname="basename"value="exceptions"/></bean><!-- lets inject the above MessageSource into this POJO --><beanid="example"class="com.foo.Example"><propertyname="messages"ref="messageSource"/></bean></beans>

The resulting output from the invocation of the execute() method will be…​

The userDao argument is required.

With regard to internationalization (i18n), Spring’s various MessageSource
implementations follow the same locale resolution and fallback rules as the standard JDK
ResourceBundle. In short, and continuing with the example messageSource defined
previously, if you want to resolve messages against the British (en-GB) locale, you
would create files called format_en_GB.properties, exceptions_en_GB.properties, and
windows_en_GB.properties respectively.

Typically, locale resolution is managed by the surrounding environment of the
application. In this example, the locale against which (British) messages will be
resolved is specified manually.

# in exceptions_en_GB.properties
argument.required=Ebagum lad, the {0} argument is required, I say, required.

You can also use the MessageSourceAware interface to acquire a reference to any
MessageSource that has been defined. Any bean that is defined in an
ApplicationContext that implements the MessageSourceAware interface is injected with
the application context’s MessageSource when the bean is created and configured.

Note

As an alternative to ResourceBundleMessageSource, Spring provides a
ReloadableResourceBundleMessageSource class. This variant supports the same bundle
file format but is more flexible than the standard JDK based
ResourceBundleMessageSource implementation. In particular, it allows for reading
files from any Spring resource location (not just from the classpath) and supports hot
reloading of bundle property files (while efficiently caching them in between). Check
out the ReloadableResourceBundleMessageSource javadocs for details.

6.15.2 Standard and Custom Events

Event handling in the ApplicationContext is provided through the ApplicationEvent
class and ApplicationListener interface. If a bean that implements the
ApplicationListener interface is deployed into the context, every time an
ApplicationEvent gets published to the ApplicationContext, that bean is notified.
Essentially, this is the standard Observer design pattern.

Tip

As of Spring 4.2, the event infrastructure has been significantly improved and offer
an annotation-based model as well as the
ability to publish any arbitrary event, that is an object that does not necessarily
extend from ApplicationEvent. When such an object is published we wrap it in a
PayloadApplicationEvent for you.

Spring provides the following standard events:

Table 6.7. Built-in Events

Event

Explanation

ContextRefreshedEvent

Published when the ApplicationContext is initialized or refreshed, for example,
using the refresh() method on the ConfigurableApplicationContext interface.
"Initialized" here means that all beans are loaded, post-processor beans are detected
and activated, singletons are pre-instantiated, and the ApplicationContext object is
ready for use. As long as the context has not been closed, a refresh can be triggered
multiple times, provided that the chosen ApplicationContext actually supports such
"hot" refreshes. For example, XmlWebApplicationContext supports hot refreshes, but
GenericApplicationContext does not.

ContextStartedEvent

Published when the ApplicationContext is started, using the start() method on the
ConfigurableApplicationContext interface. "Started" here means that all Lifecycle
beans receive an explicit start signal. Typically this signal is used to restart beans
after an explicit stop, but it may also be used to start components that have not been
configured for autostart , for example, components that have not already started on
initialization.

ContextStoppedEvent

Published when the ApplicationContext is stopped, using the stop() method on the
ConfigurableApplicationContext interface. "Stopped" here means that all Lifecycle
beans receive an explicit stop signal. A stopped context may be restarted through a
start() call.

ContextClosedEvent

Published when the ApplicationContext is closed, using the close() method on the
ConfigurableApplicationContext interface. "Closed" here means that all singleton
beans are destroyed. A closed context reaches its end of life; it cannot be refreshed
or restarted.

RequestHandledEvent

A web-specific event telling all beans that an HTTP request has been serviced. This
event is published after the request is complete. This event is only applicable to
web applications using Spring’s DispatcherServlet.

You can also create and publish your own custom events. This example demonstrates a
simple class that extends Spring’s ApplicationEvent base class:

To publish a custom ApplicationEvent, call the publishEvent() method on an
ApplicationEventPublisher. Typically this is done by creating a class that implements
ApplicationEventPublisherAware and registering it as a Spring bean. The following
example demonstrates such a class:

At configuration time, the Spring container will detect that EmailService implements
ApplicationEventPublisherAware and will automatically call
setApplicationEventPublisher(). In reality, the parameter passed in will be the Spring
container itself; you’re simply interacting with the application context via its
ApplicationEventPublisher interface.

To receive the custom ApplicationEvent, create a class that implements
ApplicationListener and register it as a Spring bean. The following example
demonstrates such a class:

Notice that ApplicationListener is generically parameterized with the type of your
custom event, BlackListEvent. This means that the onApplicationEvent() method can
remain type-safe, avoiding any need for downcasting. You may register as many event
listeners as you wish, but note that by default event listeners receive events
synchronously. This means the publishEvent() method blocks until all listeners have
finished processing the event. One advantage of this synchronous and single-threaded
approach is that when a listener receives an event, it operates inside the transaction
context of the publisher if a transaction context is available. If another strategy for
event publication becomes necessary, refer to the JavaDoc for Spring’s
ApplicationEventMulticaster interface.

The following example shows the bean definitions used to register and configure each of
the classes above:

Putting it all together, when the sendEmail() method of the emailService bean is
called, if there are any emails that should be blacklisted, a custom event of type
BlackListEvent is published. The blackListNotifier bean is registered as an
ApplicationListener and thus receives the BlackListEvent, at which point it can
notify appropriate parties.

Note

Spring’s eventing mechanism is designed for simple communication between Spring beans
within the same application context. However, for more sophisticated enterprise
integration needs, the separately-maintained
Spring Integration project provides
complete support for building lightweight,
pattern-oriented, event-driven
architectures that build upon the well-known Spring programming model.

Annotation-based Event Listeners

As of Spring 4.2, an event listener can be registered on any public method of a managed
bean via the EventListener annotation. The BlackListNotifier can be rewritten as
follows:

As you can see above, the method signature actually infer which even type it listens to. This
also works for nested generics as long as the actual event resolves the generics parameter you
would filter on.

If your method should listen to several events or if you want to define it with no
parameter at all, the event type(s) can also be specified on the annotation itself:

It is also possible to add additional runtime filtering via the condition attribute of the
annotation that defines a SpEL expression that should match to actually invoke
the method for a particular event.

For instance, our notifier can be rewritten to be only invoked if the test attribute of the
event is equal to foo:

Each SpEL expression evaluates again a dedicated context. The next table lists the items made
available to the context so one can use them for conditional event processing:

Table 6.8. Event SpEL available metadata

Name

Location

Description

Example

Event

root object

The actual ApplicationEvent

#root.event

Arguments array

root object

The arguments (as array) used for invoking the target

#root.args[0]

Argument name

evaluation context

Name of any of the method arguments. If for some reason the names are not available
(e.g. no debug information), the argument names are also available under the #a<#arg>
where #arg stands for the argument index (starting from 0).

#blEvent or #a0 (one can also use #p0 or #p<#arg> notation as an alias).

Note that #root.event allows you to access to the underlying event, even if your method
signature actually refers to an arbitrary object that was published.

If you need to publish an event as the result of processing another, just change the
method signature to return the event that should be published, something like:

Generic Events

You may also use generics to further define the structure of your event. Consider an
EntityCreatedEvent<T> where T is the type of the actual entity that got created. You
can create the following listener definition to only receive EntityCreatedEvent for a
Person:

Due to type erasure, this will only work if the event that is fired resolves the generic
parameter(s) on which the event listener filters on (that is something like
class PersonCreatedEvent extends EntityCreatedEvent<Person> { …​ }).

In certain circumstances, this may become quite tedious if all events follow the same
structure (as it should be the case for the event above). In such a case, you can
implement ResolvableTypeProvider to guide the framework beyond what the runtime
environment provides:

This works not only for ApplicationEvent but any arbitrary object that you’d send as
an event.

6.15.3 Convenient access to low-level resources

For optimal usage and understanding of application contexts, users should generally
familiarize themselves with Spring’s Resource abstraction, as described in the chapter
Chapter 7, Resources.

An application context is a ResourceLoader, which can be used to load Resources. A
Resource is essentially a more feature rich version of the JDK class java.net.URL,
in fact, the implementations of the Resource wrap an instance of java.net.URL where
appropriate. A Resource can obtain low-level resources from almost any location in a
transparent fashion, including from the classpath, a filesystem location, anywhere
describable with a standard URL, and some other variations. If the resource location
string is a simple path without any special prefixes, where those resources come from is
specific and appropriate to the actual application context type.

You can configure a bean deployed into the application context to implement the special
callback interface, ResourceLoaderAware, to be automatically called back at
initialization time with the application context itself passed in as the
ResourceLoader. You can also expose properties of type Resource, to be used to
access static resources; they will be injected into it like any other properties. You
can specify those Resource properties as simple String paths, and rely on a special
JavaBean PropertyEditor that is automatically registered by the context, to convert
those text strings to actual Resource objects when the bean is deployed.

The location path or paths supplied to an ApplicationContext constructor are actually
resource strings, and in simple form are treated appropriately to the specific context
implementation. ClassPathXmlApplicationContext treats a simple location path as a
classpath location. You can also use location paths (resource strings) with special
prefixes to force loading of definitions from the classpath or a URL, regardless of the
actual context type.

You can create ApplicationContext instances declaratively by using, for example, a
ContextLoader. Of course you can also create ApplicationContext instances
programmatically by using one of the ApplicationContext implementations.

You can register an ApplicationContext using the ContextLoaderListener as follows:

The listener inspects the contextConfigLocation parameter. If the parameter does not
exist, the listener uses /WEB-INF/applicationContext.xml as a default. When the
parameter does exist, the listener separates the String by using predefined
delimiters (comma, semicolon and whitespace) and uses the values as locations where
application contexts will be searched. Ant-style path patterns are supported as well.
Examples are /WEB-INF/*Context.xml for all files with names ending with "Context.xml",
residing in the "WEB-INF" directory, and /WEB-INF/**/*Context.xml, for all such files
in any subdirectory of "WEB-INF".

6.15.5 Deploying a Spring ApplicationContext as a Java EE RAR file

It is possible to deploy a Spring ApplicationContext as a RAR file, encapsulating the
context and all of its required bean classes and library JARs in a Java EE RAR deployment
unit. This is the equivalent of bootstrapping a standalone ApplicationContext, just hosted
in Java EE environment, being able to access the Java EE servers facilities. RAR deployment
is more natural alternative to scenario of deploying a headless WAR file, in effect, a WAR
file without any HTTP entry points that is used only for bootstrapping a Spring
ApplicationContext in a Java EE environment.

RAR deployment is ideal for application contexts that do not need HTTP entry points but
rather consist only of message endpoints and scheduled jobs. Beans in such a context can
use application server resources such as the JTA transaction manager and JNDI-bound JDBC
DataSources and JMS ConnectionFactory instances, and may also register with the
platform’s JMX server - all through Spring’s standard transaction management and JNDI
and JMX support facilities. Application components can also interact with the
application server’s JCA WorkManager through Spring’s TaskExecutor abstraction.

For a simple deployment of a Spring ApplicationContext as a Java EE RAR file: package
all application classes into a RAR file, which is a standard JAR file with a different
file extension. Add all required library JARs into the root of the RAR archive. Add a
"META-INF/ra.xml" deployment descriptor (as shown in SpringContextResourceAdapter's
JavaDoc) and the corresponding Spring XML bean definition file(s) (typically
"META-INF/applicationContext.xml"), and drop the resulting RAR file into your
application server’s deployment directory.

Note

Such RAR deployment units are usually self-contained; they do not expose components to
the outside world, not even to other modules of the same application. Interaction with a
RAR-based ApplicationContext usually occurs through JMS destinations that it shares with
other modules. A RAR-based ApplicationContext may also, for example, schedule some jobs,
reacting to new files in the file system (or the like). If it needs to allow synchronous
access from the outside, it could for example export RMI endpoints, which of course may
be used by other application modules on the same machine.

6.16 The BeanFactory

The BeanFactory provides the underlying basis for Spring’s IoC functionality but it is
only used directly in integration with other third-party frameworks and is now largely
historical in nature for most users of Spring. The BeanFactory and related interfaces,
such as BeanFactoryAware, InitializingBean, DisposableBean, are still present in
Spring for the purposes of backward compatibility with the large number of third-party
frameworks that integrate with Spring. Often third-party components that can not use
more modern equivalents such as @PostConstruct or @PreDestroy in order to remain
compatible with JDK 1.4 or to avoid a dependency on JSR-250.

This section provides additional background into the differences between the
BeanFactory and ApplicationContext and how one might access the IoC container
directly through a classic singleton lookup.

6.16.1 BeanFactory or ApplicationContext?

Use an ApplicationContext unless you have a good reason for not doing so.

Because the ApplicationContext includes all functionality of the BeanFactory, it is
generally recommended over the BeanFactory, except for a few situations such as in
embedded applications running on resource-constrained devices where memory consumption
might be critical and a few extra kilobytes might make a difference. However, for
most typical enterprise applications and systems, the ApplicationContext is what you
will want to use. Spring makes heavy use of the BeanPostProcessor extension point (to effect proxying and so on). If you use only a
plain BeanFactory, a fair amount of support such as transactions and AOP will not take
effect, at least not without some extra steps on your part. This situation could be
confusing because nothing is actually wrong with the configuration.

The following table lists features provided by the BeanFactory and
ApplicationContext interfaces and implementations.

Table 6.9. Feature Matrix

Feature

BeanFactory

ApplicationContext

Bean instantiation/wiring

Yes

Yes

Automatic BeanPostProcessor registration

No

Yes

Automatic BeanFactoryPostProcessor registration

No

Yes

Convenient MessageSource access (for i18n)

No

Yes

ApplicationEvent publication

No

Yes

To explicitly register a bean post-processor with a BeanFactory implementation,
you need to write code like this:

DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
// populate the factory with bean definitions// now register any needed BeanPostProcessor instances
MyBeanPostProcessor postProcessor = new MyBeanPostProcessor();
factory.addBeanPostProcessor(postProcessor);
// now start using the factory

To explicitly register a BeanFactoryPostProcessor when using a BeanFactory
implementation, you must write code like this:

In both cases, the explicit registration step is inconvenient, which is one reason why
the various ApplicationContext implementations are preferred above plain BeanFactory
implementations in the vast majority of Spring-backed applications, especially when
using BeanFactoryPostProcessors and BeanPostProcessors. These mechanisms implement
important functionality such as property placeholder replacement and AOP.

6.16.2 Glue code and the evil singleton

It is best to write most application code in a dependency-injection (DI) style, where
that code is served out of a Spring IoC container, has its own dependencies supplied by
the container when it is created, and is completely unaware of the container. However,
for the small glue layers of code that are sometimes needed to tie other code together,
you sometimes need a singleton (or quasi-singleton) style access to a Spring IoC
container. For example, third-party code may try to construct new objects directly (
Class.forName() style), without the ability to get these objects out of a Spring IoC
container.If the object constructed by the third-party code is a small stub or proxy,
which then uses a singleton style access to a Spring IoC container to get a real object
to delegate to, then inversion of control has still been achieved for the majority of
the code (the object coming out of the container). Thus most code is still unaware of
the container or how it is accessed, and remains decoupled from other code, with all
ensuing benefits. EJBs may also use this stub/proxy approach to delegate to a plain Java
implementation object, retrieved from a Spring IoC container. While the Spring IoC
container itself ideally does not have to be a singleton, it may be unrealistic in terms
of memory usage or initialization times (when using beans in the Spring IoC container
such as a Hibernate SessionFactory) for each bean to use its own, non-singleton Spring
IoC container.

Looking up the application context in a service locator style is sometimes the only
option for accessing shared Spring-managed components, such as in an EJB 2.1
environment, or when you want to share a single ApplicationContext as a parent to
WebApplicationContexts across WAR files. In this case you should look into using the
utility class
ContextSingletonBeanFactoryLocator
locator that is described in this
Spring
team blog entry.